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Simple DC AFE for ADC chips with unipolar diff inputs
iMo:
I've been searching for a simple DC AFE for a basic "Voltmeter", based on ADS12xx or LTC2xxx chips. Having some ADC chips in my junkbox I want to start to play with them as fast as possible, where the target is "6 digits voltmeter only" at this stage. The approach is KISS.
Thus the basic spec for the AFE:
1. input voltage +/- 20V max
2. input impedance >>100Meg
3. DC measurements only
4. basic input protection
5. AFE not "metrology grade" one, but it should not introduce too much noise and drift
6. supporting up to 6.5 digits with appropriate ADCs (ie. see TiN's ADS1263 review and test)
7. AFE shall feed the ADC chips with differential inputs, but unipolar voltage range (0-5V)
The basic idea is to have a chopper opamp, HV one for example.
The AFE and ADC will get floating power supplies, and the ADC's INP_N input is the Voltmeter's "ground terminal".
See below the basic concept.
When switching FB to A or B you can get x1 or x10 amplification.
When switching C to A or B you can get x1 or x0.1 attenuation.
Open issues: how to protect the input of the ADC's INP_P and INP_N.
Any hints welcomed.
PS: let us not discuss the suitability of various monolithic SD/SAR 24bits++ ADCs for "DMM" purposes here, plz.
jaromir:
Using LTC2057 to generate half of Vref seems to be overkill here. You can use something like MCP6001 powered by power supply of ADC.
JFET seems to be common choice for overvoltage protection, for its low leakage. You may need to manually select individual transistors, though.
Also, be careful with C4 and C5 selection.
MasterT:
Emitter's junctions are "no go" anywhere near signal path.
It's not clear, why AFE is necessary, seems ADC has internal PGA with 1 G impedance and 40 M with bypass. Why not use a simple resistive divider to extend input range?
iMo:
@jaromir:
TI's appnote sboa058 says Siliconix's 2n4117 claims 60fA.
Tried with MCP6021 (6001 does not simulate here).
Simulation shows a little bit worse result (1.5uV off with +/-20V input).
@MasterT: it depends on ADC chip used, the goal is >100Meg (with careful build).
MasterT:
I had negative experience using AZ OPA MCP6V28 for buffering reference voltage for 16-bits DAC. The problem was with a bias current, I mistakenly didn't follow microchip recommendation to keep input impedance seen by OPA equal:
--- Quote ---Make the resistances seen by the inputs small and
equal. This minimizes the output offset caused by the
input bias currents.
--- End quote ---
p.24
Looking at LT2057 ds, I see that LT recommends just exactly opposite:
--- Quote ---Injection currents from the two inputs are of equal magni-
tude but opposite direction. Therefore, input bias current
effects due to injection currents will not be canceled by
placing matched impedances at both inputs.
--- End quote ---
p.19
Doesn't matter if it's in the same direction or not, but I'd not use AZ in any high input (100 M) impedance application. On the same page 19:
--- Quote ---When these small
current pulses, typically about 0.7nA RMS , interact with
source impedances or gain setting resistors, the resulting
voltage spikes are amplified by the closed loop gain.
--- End quote ---
V = R x I = 100 M x 0.7nA = 70 mV.
It makes AZ even worse than LM358 with 10 mV offset, and LM358 is preferable since offset is pure DC no HF glitches, and easily subtracted by uCPU at start-up re-calibration subroutine.
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