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I want to amplify a 0-20mV signal from a sensor to 0-2.048V in order to feed it to an ADC (with a reference voltage of 2.048V). After I will input the digital signal to a microcontroller. I need to be able to detect a change in the 0-20mV signal with an accuracy of 5µV. Thus after amplification to 0-2.048V (using a gain of 102) an accuracy of 0.5mV is needed. My sensor signal changes very slowly and I only need to take one sample/measurement per second.

I have chosen this instrumentation amplifier for amplification (differentially) AD8237: Datasheet

Now in order to get a true ground output swing (as the signal is in the 0-20mV range initially) I want to use the LM7705: Datasheet

By powering the positive supply Pin of the AD8237 with 3.3V and the GND Pin with this little negative ground from the LM7705 I should be able to get an output swing to zero volts (thus, use the whole range of my ADC). The LM7705 Negative Bias Generator seems to be made exactly for this purpose. I tried to check this idea with the Diamond Plot tool by Analog Devices and everything seems to be in the expected and valid range: Click to view my configuration

Also regarding the accuracy, the biggest issue I see is the input offset of 75µV of the in-amp, but I should be able to correct the value later in software (calibration). Do you see problems regarding the small negative ground supply for this instrumentation aplifier? I couldnt find anything against it in the datasheet. (To the ref pin I would connect the normal ground of 0V). Is that ok?

Further, I chose this 18Bit ADC (MCP3422): Datasheet

Would also power it with 3.3V and connect the in-amp output directly to this ADC (single ended). It has an on-board reference voltage of 2.048V.

Can I connect the in-amp output directly to this ADC? Do you see big problems with this approach or ADC and its accuracy (so I can meet my specified minimum requirements)?

I appreciate every hint/feedback/help I get from you, thank you!

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  • \$\begingroup\$ What temperature range will you operate the InAmp over? \$\endgroup\$ – Andy aka Dec 14 '18 at 16:22
  • \$\begingroup\$ @Andyaka: It will be used in a circuit that is placed in a not heated box/room. So over a whole year the temperatures could possibly vary between -5°C to 40°C. \$\endgroup\$ – Henry Dec 14 '18 at 16:26
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    \$\begingroup\$ The input offset voltage on the AD8237 drifts at 0.3 uV per degC hence you could see a drift of up to 13.5 uV equivalent at the input over this temperature range. \$\endgroup\$ – Andy aka Dec 14 '18 at 16:28
  • \$\begingroup\$ @Andyaka: Thanks for pointing that out. I think the specified temp. range should really be the worst case (possibly going to be way smaller). If I take a max. temp. drift of lets say 15µV, that would cost me being 3 readings/resolution steps off (in absolute worst case). I took this INA because it has very low gain drift/error and accepts my very low common mode votlage (can even go below 0 volts). So far I did not find any other INA that meets all this requirements. Do you see additional issues except the temp. drift in my approach? Any thoughts on the LM7705 or the ADC? Thank you! :) \$\endgroup\$ – Henry Dec 14 '18 at 16:40
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    \$\begingroup\$ The 7705 seems cool and no, I didn't see any other problems except your fixation on reaching exactly full scale with 20 mV as an input; don't do this because gain errors and offset errors in the ADC will undo you. \$\endgroup\$ – Andy aka Dec 14 '18 at 16:44
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I ran this signal chain thru Signal Chain Explorer; you need a strong filter before the ADC [see notes at end, on further improvement of SNR]

enter image description here

Without the LowPass RC filter, the SignalNoiseRatio drops from 78dB to 58dB, because of random noise from the Instrumentation Amplifier. I assumed 70 nanoVolts/rtHz noise density.

And if I enable the various interferers (the top right Gargoyles button), the SNR also drops, to 57dB; the dominant interferer is the (default) nearby switching regulator, located only 1cm from your signal Chain, and trashing the PCB trace from the LPF into the ADC.

Here is a textual presentation of the 4 Gargoyles, using the defaults from the 4 (editable) tables that define the interferers {for EFI and HFI, slewrate is the primary property that causes interference; plus distance}

enter image description here

Notice the RC LPF right before the ADC. The Gargoyles part of the simulation uses the mechanical awareness of the SCE tool, where PCB trace defaults are 10mm trace with dual 2mm diameter vias and trace width of 1mm, and 1.5mm height (1/16" inch) above the backside Ground trace (or plane). The area of the trace is the vulnerability to electric fields. The height (1.5mm) and the length (14mm) define a LOOP AREA, which is the vulnerable region for magnetic fields.

I picked 1,500 ohms and 1uF to create the 100Hz Low Pass Filter. By default, the 1uF capacitor is 14mm away from the ADC input pin. That distance is the primary victim of the Switching Power Supply magnetic field.

I suggest: (1) place the 1uF capacitor right at the ADC input pin. (2) don't have any Switching Power Supplies near the LPF+ADC PCB trace. (3) be aware that interference into the sensor/opamp trace may be so high that the OpAmp is driven into non-linearity, and you get surprising errors.

Be aware the ADC draws some input current as the input sampling capacitor charges up to your (2.048 volts max) input voltage. That average current will cause a voltage drop across the 1,500 ohm resistor.

By the way, the tool is free for download at: robustcircuitdesign.com

Here is the amplitude error (only the LPF is enabled); note the OpAmp stage and the ADC stage are de-selected.

enter image description here

The error (LPF only; LPF has F3dB of 100Hertz) is 0.0004 dB, where a dB for voltage_ratio is 12% per dB. The error is approximately 12% * 0.0004 or about 50 parts per million, or 1/20,000

The opamp gain (click on the OpAmp stage, then click "Show Open/Closed Response", is down 0.1% at 1Hz. Are you concerned with that, or with settling accuracy?

Note the SNR (with the 4 interferers enabled by the Globally-effective "Gargoyles" button activated), is 57dB.

Again, with Gargoyles OFF, and the 100 Hertz filter checked ON, and the OpAmp and ADC also checked ON, the predicted SNR is 78dB and ENOB is 12.7 bits.

Just keep any Switching Power Supply at least 20dB (10X) further away than the default 10mm distance used in the HFI Gargoyle table; click on "HFI" to see that table; have at least 100mm (4") from any Switching Supply and your Signal Chain, at least from the PCB trace of LPF to ADC.

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edit, to present more effective low-pass-filter

schematic

simulate this circuit – Schematic created using CircuitLab

In the second screen shot, there is a text-window summary of the strength of the interferers:

Magnetic Field Induced HFI is 930 microvolts

Power Supply PSI, entering the amplifier (and the ADC) VDD pins, at 60 and 120 Hz, with a moderate PowerSupplyRejection of 80dB at the lower frequencies, and with 10 milliVolts, is 100 microVolts

Electric field induced EFI is dominated by spikes on the power line (the power cord runs thru the region of the Signal Chain), is 37 microVolts.

Lastly, Ground Plane Interferer assumes 0.1 amp of trash, at 10MHz, (for a Switching Supply) flows thru 2 squares of the plane, is 12 microVolts.

In the first screen shot, look on right hand side, and read the computed Quantization standard deviation as 2.2 microVolts. That is the ADC floor.

Thus the next Gargoyle of importance is the POWER SUPPLY RIPPLE, assumed to be 10 milliVolts. Add resistors (10 ohms, to cause only a small DC voltage drop) and a large capacitor, to drop the 120/60Hz ripple by 6 or 10 dB. This requires a F3dB of? 30Hz, which requires a Time Constant 1/(6.28 * 30) or about 5 milliSeconds. Thus a 470uF capacitor will do. And the ADC may need VDD filtering as well. Read that datasheet.

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    \$\begingroup\$ @cap Henry Just keep any switching-power-supply at least 4" from the final PCB trace,that runs from the 100Hz LPF to the ADC. Best way is to place the 1uF cap right by the ADC, and ground the cap to AnalogGround of the ADC. \$\endgroup\$ – analogsystemsrf Dec 16 '18 at 23:01
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    \$\begingroup\$ Switching power supplies (and the black bricks) are death to high SNR systems. One way to test is to make a 1" square loop at end of a coax cable, then run that 1" square around the (some) chosen switcher, and measure the trash level on your scope. Then scale down the measured trash level, to the default loop area used in Signal Chain Explorer (14mm long and 1.5mm high). For some magnetic field trash generators, the field drops off as 1/Distance; for other trash generators, the field drops off as 1/Distance^2. The math in SCE HFI Gargoyle assumes a simple wire-loop model, and 1/Distance. \$\endgroup\$ – analogsystemsrf Jan 13 at 3:01
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    \$\begingroup\$ You can lower the RC lpf to 10 Hertz or 20Hz or 5Hz, if you want. You will get slower settling at the lower bandwidths. At 1 neper per time constant, and 6.28 TAU per hertz, the 5 Hertz filter allows 5 * 6.28 = 31.5 tau, or 31.5 * 8.6dB = 250+ dB of settling accuracy, if you wait for one second after the sensor output changes. \$\endgroup\$ – analogsystemsrf Jan 13 at 3:04
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    \$\begingroup\$ @ Henry You must also keep the MCU away, and not bring any MCU lines (Sleep, Mode, Select) near. These couple quite well thru the air, as EFI. Install the tool, and examine the EFI database (just click the "EFI"). Your next Gargoyle is the PowerSupply VDD ripple, assumed to ONLY BE 60/120Hz. Install 10 ohms and 470uF in VDD traces, IF you have over 1milliVolt of ripple. \$\endgroup\$ – analogsystemsrf Jan 13 at 16:36
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    \$\begingroup\$ Henry If the ADC input RC filter is fast enough, then there will be extremely low DC error across the resistor. On Jan 13, I computed 250dB of settling accuracy if a 5Hz filter is used. Regarding L+C filters, the L needs to be dampened with a parallel resistor; also, at 5Hz, the L will be enormous, and very susceptible to external magnetic fields; hence I prefer to use a tiny R instead of a large and expensive L. \$\endgroup\$ – analogsystemsrf Jan 20 at 12:43

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