Techniques that Reduce System Noise in ADC Circuits · LM 7805 A5 2.5V Reference R3 = 300kΩ, R4 = 100kΩ, RG = 4020Ω, (+/-1%) MCP602 = Single Supply, CMOS, dual op amp MCP3201 =

Microchip Technology Inc.WebSeminar: Feb. 18, 2004

Techniques that Reduce System Noise in ADC Circuits Page 1

© 2004 Microchip Technology Incorporated. All Rights Reserved. ADC System Noise 1

Techniques that ReduceSystem Noise in ADC

Circuits

My name is Bonnie baker and I am an engineer in Microchip’s Analog division.




Agenda

� Consequences of Designing with no Regardto Noise

� Areas to Concentrate on� Device Noise� Conducted Noise� Radiated Noise

� Test Results of a Good Design

We’re going to talk about techniques that reduce system noise in ADC circuits,which really is kind-of a fun topic. We’re going to go from the circuit and dig intothe board level and I’ll show you what works, and what doesn’t work, and howto do your corrections if things aren’t going your way. So, it will be a very quickafternoon for 20 minutes, but I think we’re going to have a lot of fun here.

Starting out, we’re going to talk about the consequences of designing with noregard to noise. I think all of us, at one time or another, have been in enoughhurry by our management to throw things on the board, just cross your fingersand hope it works right. We’ll talk about some of these issues. And then we’llback up and say, ‘well, ok, where did we go wrong; what were we thinkingabout and concentrating on’.

From there we’ll go into device noise, conductive noise and radiated noise andtalk about the impact of these various noise sources. We’ll finish off thisseminar with test results of a good design. Now, a good design will bedesigned from the same format or premise of the original design that you willsee. The original design really doesn’t perform very well. So, it will be fun to seehow we can just kind-of tweak the design in and get the noise out of thesystem.




Long Traces: Antenna

� Trace going into 10-bit ADC input is longerthan a few inches

Emitted Noise

Ok, well, this is a common conversation that I have with people, customers andeven engineers here at Microchip. A person will walk up to me and say, ‘youknow, I have this 10-bit ADC board and I’m not getting 10 bits. I don’tunderstand it. The last LSBs of my 10-bit data are just kind of flipping all overthe place. Dithering back and forth. What seems to be wrong?’

‘Well,’ I would say, ‘let’s have some fun with this. Let’s look at your board andsee what you are doing.’ So glancing at a board, we might see that a tracegoing into the A/C converter is long, longer than we should really becomfortable with. What does that do? What might be introduced into thattrace? It might be susceptible to emitted noise. I don’t know, but that is a redflag.




Low-Pass Filter Missing

� Traces going into ADC inputs longer than afew inches

� No anti-aliasing (or Low-Pass) Filter at theinput of the ADC

ADC

C1

C2R5

R6

PIC

16C

623

++++

−−−−

1/4 ofMCP6024

2nd orderLow Pass Filter

So then you go down and you continue to look at the board together. We noticethat there is no anti-aliasing filter at the input of the A/D converter. Well, this isprobably a problem because the A/D converter will religiously convert higherfrequency noises into the system and it might produce the noise that you’reseeing at the output of the converter in your circuit. So, ok, let’s put that downas something to take a look at.




Discontinuous Ground Plane

� Traces going into ADC inputs longer than afew inches

� No anti-aliasing (or Low-Pass) Filter at theinput of the ADC

� Interrupted Ground Plane on the Back Sideof the Board

Ground

Finally, we’ll take the board and flip it over on the backside. Now a lot ofcustomers that I have worked with don’t have a ground plane.That’s really adanger-danger situation. But with this particular board that we looked at, it hada ground plane. The engineer said to me, ‘I did put in the ground plane’. But Iwant you to look at some lines that I’m going to draw on your slide right herefrom the ground connector/pin, and what the current has to do in order to getback to that pin. Imagine your ground pin is pin 8. That current has to goaround in that direction, or it has to come on the other side. I’m drawing thisright now and it will show up in a second.

Well, that is interesting because that is not a solid ground plane. The areaswhere you think you have a solid ground plane, where these pins that werethrown into the board, is actually a complete block or wall. This is because youhave vias going through the board. Another wall or block would be this parallelbus that is going across your board. So we went into it saying ‘ I have a groundplane’, and then you take a look at it and say, ‘gee, maybe I don’t because I justcut it into pieces’. So, those are some issues that I usually look at when I firstlook at a board.



Application Circuit

CS

VDD = 5V

MCP3201

LCL- 816G

+

-

VDD

+

-

R3

R3

RG

R4

R4

SCLKDOUT

R1 R2

R1R2

VDD

PIC

16C

623

VDD

1/2 ofMCP602

1/2 ofMCP602

MCP1525

A1

A2

A4

Two-op-ampInstrumentation Amplifier

Wall Wart

9V DC out

LM7805

A52.5V Reference

R3 = 300kΩΩΩΩ, R4 = 100kΩΩΩΩ, RG = 4020ΩΩΩΩ, (+/-1%)MCP602 = Single Supply, CMOS, dual op ampMCP3201 = 12-bit, A/D SAR Converter

A6

Now, let’s look at one of my boards that I designed and we’ll see what the results are. This is kind-of acomplex circuit that we’re looking at but we’re going to continue to use this through the seminar. I’ll gothrough it one bit at a time, so you can understand how it works. On the front end I have a sensor and it is aWheatstone bridge sensor that is actually a load cell. The LCL-816 that I got out of the Omega data bookand they shipped it to me. Following that the load cell is connected to the instrumentation amplifier. Thoselines go into the instrumentation amplifier in this manner. The instrumentation amplifier has two kinds youcan live with. The signal goes into this configuration, which happens to be a 2 op-amp instrumentationamplifier. It is surrounded by resistors, 5 resistors as a matter of fact. Then, at one of the pins, since this asingle supply application, there is a voltage reference going into that instrumentation amplifier. Ok, that’sgreat. Then the signal leaves the instrumentation amplifier and goes straight into the A/D converter. If youare paying attention to what I am talking about, you’re probably going to say, ‘gee, Bonnie, you left out theanti-aliasing filter’ and you’re correct. But let’s just say, ‘I don’t need that because, I’m working in my lab(ina low noise environment) and that’s not going effect me at all’.

There is another area that we need to look at and that is up along the top of slide. That is where our powercoming out of the wall. This is an often, ignored place to take a look for noise. You take the power out ofthe wall with what I call a wall-wart and the output of that little box is 9 volts DC.It goes into a linearregulator. Iinitially, you would say, ‘gee, my linear regulator will fix all. It will get rid of all of the nonsenseand now I have a linear power supply’. Around the linear regulator there are the appropriate capacitors andI have designed this per the data sheet recommendations for the LM7805. That power then goes down andsupplies power to the A/D converter, the op-amp, the regulator, the sensor and the pic micro.

So, that looks good. I’ve got a good circuit. There are some things to pay attention to as we go forward. Ielected to put these resistors (and I’m underlying them right now) into the circuit; 300kΩ, 100kΩ and 4kΩaround the instrumentation amplifier. My justification for these values is, ‘well, that is what I found in the laband it works just fine for me’. The next thing I’ve elected to do is put in this amplifier, the MCP602. Theamplifier is really a good amplifier, and a lot of our customers buy it and use it; and I so I thought well let’sgo with that and see how that works. Then I used the 12-bit A/D converter. As we go through thisdiscussion, I will talk about these circuits and whether or not they are a good idea for my system or not.But, there we go.




Application Board #1: Top side

HIGHER VALUE RESISTORS / NOISY AMPLIFIER /NO BY-PASS CAPS / NO LP FILTER / NO SUPPLY CHOKE

So, I go into the lab and I put it on a pc board. The pc board implementation looks somethinglike this. I want to point some things out to you in this slide. First of all, the sensor that I amusing is right here; that is a load-cell sensor and it is a resistor sensor hanging out over the edgeof the board. I can excite it by putting some weights out there and get some small voltages togo across it.Then the signal goes from that sensor to this chip, which is the instrumentation amplifier.Thatinstrumentation amplifier is done with (as you saw in the diagram) the op-amp, and someresistors and a voltage reference, right there; they are all grouped together. From there thesignal goes to the A/D converter. There is no filter. I have a buffer here, but that’s not doinganything. Then the signal goes into the A/D converter. Fantastic. I have a great board; thelayout looks ok to me. I’m pulling the power off onto the board at this point in the layout. Andit’s coming from another control board that I have that I’m not showing you.So, what do I have here; some things to pay attention to. First of all I have the high-valueresistors, and we don’t know that now (and I haven’t explained it) but my amplifiers are a little bitnoisier than I can tolerate in this particular circuit. Something else that I haven’t talked about yetis, there are no by-pass capacitors in the circuit. I have a lot of people who tell me that they arenot necessary and quite frankly, with digital circuits a lot of times you can get away with notputting in by-pass caps. But not in the analog domain.Going on, there is no power-supply filter or no supply choke. In other words, my supply voltageis coming from the wall; going through a switching wall-wart is what I call it, into a linearregulator and then into my circuit. I am not clamping down on the noise that is in the powersupply. For the most part, I thought I didn’t have to because I have a linear regulator in thecircuit.




Application Board #1 Bottom side

NO GROUND PLANE

I want you to see the backside of the board, and this is it. I flipped over theboard and this slide shows the backside of the board. Here you see the sensor,right here, there is the sensor so you can see how it is flipped over. There is noground plane anywhere to be found. All of the traces that you see on thebackside of this board are primarily ground traces. I’ve gone to great lengths, toinclude all of these ground traces and connect everything properly. But there isno ground plane and that is an important thing to pay attention to in yourdesign.




Num

ber

of O

ccur

renc

es

70

60

50

40

30

20

100

80

90

Output Code of 12-bit A/D Converter2960 2970 2980 2990

Code Widthof Noise = 44

(total samples = 1024)

Application Board #1Test Results

Now, let’s look at the results. I have taken the data off of the board and put itinto the computer. I generated a histogram from a group of the data that I’vesampled. The data was sampled at 10,000 samples per second. I took 1,024samples off the board. So, out of those 1,024 samples, you’ll look at the x-axishere. The x-axis is the code out of the A/D converter and this is showing 2955up to 3000; that is on the x-axis. On the y-axis, are a number of occurrences ofeach code of the 1024 taken. I’ve separated it into a histogram format. Theinput signal is actually the DC signal; there is no noise coming from that inputsignal. So, coming out of this board, supposedly I should see is a DC signal,constant DC signal, but I am actually seeing 44 codes.

If I go through the calculations and try to decide what kind of converter I have,or what kind of bits that I am getting out of my converter, I would guess that itwould be 6 or 7 bits (I don’t know, it has been a while since I’ve calculated). ButI am certainly not getting the 12-bit result. Maybe we need to go back and talkabout some things. And here we go.




Low Noise Design� Device Noise - Created by the devices

� Resistors - Reduce Values were possible� Op amps - Use Lower Noise Amplifiers� Power Supply - Replace Switching Devices

� Emitted Noise - Externally Injected� Layout - Keep analog and digital Separate� Environment - Shield or change orientation

� Conducted Noise - In the Circuit Traces� Use a Continuous Ground Plane� Filter Signal traces� Filter Supply traces

We are going to go back and examine device noise, emitted noise andconducted noise.

Device noise is created by the particular device, whether it is passive or active.For instance, a resistor generates noises. An inside an operational amplifier,from the transistors and the resistors, generates noise. Additionally a powersupply that is switching has obvious noise that is generating power supply line.

Emitted noise is another type of noise that we have to consider. Emitted noiseis noise that radiates into your circuit, one way or the other. The most commonradiated noise is the noise that goes across your board and I’ll show you howthat works.

Finally, the third noise source that (you know I always consider when I checkinto these things) is conducted noise. Conducted noise is either device noise oremitted noise that is now in your circuit traces. You might say, ‘well, get rid ofall of your device noise and the emitted noise. Then you have a clean circuit’.

There is some truth that, but you may not be able to get rid of that conductivenoise. In other words, you will have to put resistors in your circuit, you will haveto put your amplifiers in your circuit, and you probably will need a power supplysome place. And so the trick is, now you know the noise is there, how do youget rid of it. We’ll go into some details on how to tackle that problem.




Device Noise� Resistors

� Johnson or Thermal Noise� VRN = 4KTR(BW) {Vrms}

K = Boltzman’s Constant = 1.38e -23 JK-1T = Temperature in KelvinR = Resistance in Ohms(BW) = Noise Bandwidth in Hz

� 1kΩ = 4nV / √ Hz� Amplifiers

� MCP602 Specification -29nV/ Hz @ 1kHz

� MCP6022 Specification -10nV/ Hz @ 10kHz

nV/ Hz(log)

Frequency (log)

1 / f noise

BroadbandNoise

Talking about device noise first. Resistors will always generate noise. This is the formula youcan use to calculate ideal noise coming from the resistor. It is equal to the square root of 4 timesK (which is Boltzman’s constant) times temperature (which is in Kelvin) times the resistance(which you know in ohms) and times the bandwidth of interest. The result of this calculation isin volts RMS. A lot of times you will see it in nano volts RMS. But you go through thecalculations and if I don’t consider the bandwidth, say with a 1kΩ resistor, the noise from a 1kΩresistors 4 nano volts per hertz. So what is the noise from a resistor across the bandwidth of,say, 10Hz to 10,000 Hz? Well the square root of 10k is 100. So the noise is 4 nano volts times100 or 400 nano volts across that bandwidth. Now a 1kΩ resistor is rather small. You mayremember that I used higher value resistors, like hundreds of thousands of ohms.

Another place where devices generate noise is in the amplifiers. We have a couple of amplifiersin our circuit so this is an interesting discussion. The MCP602, which is the first amplifier Iused has a noise figure of merit of 29 nano volts per Hz at 1kHz. In other words, at 1kHz of thisfrequency diagram the magnitude of noise over a 1Hz bandwidth is 29 nano volts. That noisewill probably stay flat as it goes up in frequency. However, you do have that 1/f noise right hereto take into consideration. But we are not going to talk about that today, that is another seminarall together.

Taking a look at MCP6022, this amplifier has noise figure of merit of 10 nano volts per root Hzat 10KHz. Basically what that says is that this amplifier has that kind of noise at 10 nano voltsper root hertz and beyond 10kHz it stays flat. That is about a 3 x reduction in noise just from theother amplifier. Well, I am going to switch to the lower-noise amplifier in my circuit, and I’mgoing to reduce the resistors in my circuit as well. I think it is 10 x.




� Decrease “L” or Increase “d”� Put Ground Guard Between Traces

Emitted NoiseC = pF

w • L • eo • erd

Voltage IN PCB Trace

dL

Guard Trace

CoupledCurrent

I = C (amps)dV

dt

So, let’s proceed on to discuss the emitted noise. The most common emitted noise that youwill have on your board is to have two traces close together on the board, not touching, closetogether and one trace will have some kind of digital signal on it, like is shown here. Asquare wave, if you will. The other trace will be high impedance. With that high impedancethe current that is coupled from trace to trace will turn into a voltage that looks something likethis. Interesting consequence.

Basically, the capacitance from trace to trace is specified with a formula up here at the top ofthe slide. Capacitance equals the width (which is the depth of the trace) times the length(which is the length of the traces are beside each other which is right here and the distancebetween the two traces in the denominator. e0 and er are the dielectric constants of theparticular PC board.

Using that formula you can calculate what the capacitance is. From there you can determinewhat this current is and then put resistance into the formula. From there you can get thiswaveform out of it. This is really common place for noise to radiate across from trace totrace.




Conducted Noise

� Ground and Power� 50Hz or 60Hz� Ground and Supply Current Return Paths� Use continuous Ground Plane and Filter

Supply traces� Signal Path

� Digital Switching� Noise Generated by Previous Device� Use Analog Filters

Conductive noise is another issue; now, as I mentioned before, that is anoise that is in your PC board, in your tracers. It came from somewhere, youmay or may not know where it came from, but there it is and you need tofigure out how to get rid of it. Some real common places or things that causeconductive noise is this mains signal; 50 or 60hertz, depending on what partof the world you are in. And a lot of times that radiates into your circuit boardor it couples in through your power supply. Another place where you canhave this kind of conductive noise problem is your supply or spike in yourground return path.

Then, if you don’t use a continuous ground plane or filter the supply you willhave maybe this ground and power noise coming into your circuit. If you arelooking at the signal path, the digital switching that we talked about might bewhere the conductive noise has arrived. Noise generated by the previousdevice, which would imply that maybe that amplifier that generated the noise,and how do you get rid of this in your signal path, you use filters. How doyou get rid of ground and power noise? You can expand your power plane,or you can use filters to reduce that noise. But with all of these tricks, wedon’t have enough time to go into it.



12-bit Accurate Circuit ComponentsR3 = 30kΩΩΩΩ, R4 = 10kΩΩΩΩ, RG = 402ΩΩΩΩ, (+/-1%)R5 = 27.4kΩΩΩΩ, R6 = 196kΩΩΩΩ, C1 = 100nF, C2 = 470nFMCP6022 = Single Supply, CMOS, low noise, dual op ampMCP3201 = 12-bit, A/D SAR Converter

CS

VDD = 5V

MCP3201LCL-

816G

+

-

VDD

+

-

R3

R3

RG

R4

R4

C1

C2R5

R6SCLKDOUT

R1 R2

R1R2

VDD

PIC

16C

623

VDD

+

-

1/2 ofMCP6022

1/2 ofMCP6022

1/2 ofMCP6022

MCP1525

A1

A2

A3

A4

Two-op-ampInstrumentation Amplifier

2nd orderLow Pass Filter

Wall Wart

9V DC out

L1InductiveChoke

LM7805

A52.5V Reference

Application Circuit

A6

We are going to go back to the circuit that we started with. We haven’tchanged any thing; we just added a few things. And the few things that wehave added or done are in yellow.

First of all, we put in the by-pass capacitors and there is one up there. Thereare on every single chip throughout this circuit. Second of all, we havechanged the resistors and reduced them by 10 times. That is shown up at thetop of the slide here. Third of all, we added a low-pass filter. Those resistorsshow up here and the low-pass filter in the circuit is shown here. Another thingwe did is we changed the amplifier to a lower noise amplifier; from the MCP602to the MCP6022. Coming off the power supply, I know that I have a switchingsupply coming out of this wall-wart. I also know that I have something like a DCoutput, but I there is noise on top of it. Additionally, I have suspicions that thislinear regulator is not filtering out all of my noise. So I am putting in an inductivechoke to get rid of the switching noise. That turned out to be quite a criticalmove or change on the board to give me a good, low noise result.

Let’s go to the next slide and look at the actual implementation.




Application Board #2

It’s not that much different of a board, but this time I believe that it will work.Here is your sensor, again, and your signal path is going out from the sensor tothe instrumentation amplifier. You will notice the position of the chips have notchanged, but I have added that by-pass cap, right there next to the amplifier.Additionally, I have put in a low-pass filter. It is only a second order, low-passfilter, but it does a nice job.Up here, I put in the inductor at the very top. Remember, I said before that thepower was coming off of this connector here and going on to the board. Thatsignal goes off of the board and I take it into the computer.So, that is basically what I did. And let’s just look and see what happens interms of the test results.




Output Code of 12-bit A/D Converter2940 2941 2942

Num

ber

of O

ccur

renc

es

700

600500400300200100

0

800900

10001100

Code Widthof Noise = 1

(total samples = 1024)

Application Board #2Test Results

If I take 1,024 samples there is no input signal, so that is DC. 1,024 sampleswere taken at the same data rate. Nothing has changed in the environment.These data points are placed into a histogram environment. Notice that it iscompletely one code. 1,024 samples landed on one code.In the beginning I thought I didn’t have a 12-bit converter and there wassomething wrong with it. But as I cleaned up my board and cleaned up mycircuit, it turns out I have a true 12-bit converter. Nothing was wrong with theconverter. I just needed a little clean up to be done.




ConclusionWhat you Should Do

� Verify circuit devices are low-noise� Always have an uninterrupted ground plane

on board� Filter ADC with a low-pass anti-aliasing filter.� By-pass all devices properly.

� Place the capacitors as close to the power pinsof the devices as possible.

� Filter your power supply properly

In conclusion, when you try to design and really want to design it right time thefirst time, verify your circuit devices are low noise. Always, always, have an un-interrupted ground plane. Think about how the current is going to go across thatboard. If you do have to have vias always make sure that current is notinterrupted.

Always filter the A/D converter with a low-pass anti-aliasing filter. Someconverters only require an RC pair for filtering. Other converters require higherorder filters. But always make sure that filter is there. By all means by-passdevices properly, by putting the capacitors close to the power pins of the deviceas possible.

And finally, and this is a “gotcha” for a lot of people that I work with, make surethat your power supply is filtered properly.




References� Reference Material

� ADN 007, Techniques that Reduce System Noise in ADC Circuits� AN681, Reading and Using Fast Fourier Transforms (FFTs)� AN699, Anti-Aliasing, Analog Filters for Data Acquisition Systems� AN695, Interfacing Pressure Sensors to Microchip’s Analog

Peripherals� AN688, Layout Tips for 12-Bit A/D Converter Application

� High-speed Digital Design: A Handbook of Black Magic, Howard Johnsonand Martin Graham, Prentice Hall, 1993.

� Noise Reduction Techniques in Electronic Systems, Henry Ott, JohnWiley, N.Y., 1998.

� FilterLabTM Active Filter Design Software� Down-loadable at www.microchip.com� Active, op amp filters

So that concludes this seminar. There are some references you might want to go deeper intobecause I know this seminar was really quite brief and not a lot of details were given. ADN007is out and available and on our web. It talks about these techniques that I went over brieflytoday. Another application note, ADN688, refers specifically to this design in great detail. Thepressure sensor application note, ADN695, is the same as the low-cell sensory essentiallybecause it is a resistive network, just different kind of excitation. And if you are confused aboutanti-aliasing filters and why you need and how to design them, AN699 is a good source.

The other place that I personally look in the industry for references are these books here. Onewas written by Howard Johnson, the columnist that is on Electronic Design News Magazine;and the second one, Noise Reduction Techniques, by Henry Ott. The Ott book is an excellentbook. As a matter of fact it is on my desk today, and I’m refreshing my memory with a lot ofissues.

The last reference that I would recommend you consider using is the FilterLAB software. ThisFilterLAB software is designed for active filters. You can use it to develop your own filters andbuild them in your lab, right now. It is free, down load it from Microchip’s web sight. You canuse any amplifier you want to with it. We prefer you use ours, but you can use any amplifier todevelop these low-pass filters.

I guess we are at the end of this seminar. Thank you very much for your time.




Techniques that ReduceSystem Noise in ADC

Circuits

Documents

Techniques that Reduce System Noise in ADC Circuits · LM 7805 A5 2.5V Reference R3 = 300kΩ, R4 = 100kΩ, RG = 4020Ω, (+/-1%) MCP602 = Single Supply, CMOS, dual op amp MCP3201 =