(Very) High Impedance Voltmeter and ADS1115 - Page 1
Forum section: electronics / beginners

author: stcoso
author_rank: Regular Contributor
timestamp: March 24, 2019, 07:08:17 pm
content: Hi to everyone... I'm trying to build a simple high impedance (TOhm range) voltmeter and use a ADS1115 as a ADC to convert the signal. The signal I need to measure is in the range of +- 0.5VI managed to get some results using the circuit that i attached but i would like to know how it can be improved. Because the ADS1115 is usable only down to GND-0.3V i thought that i could use a resistive divider to get a 2.5V voltage to use as reference attached to one of the probes. The trimmer in the schematic is set to get a reading of 0mV when the probes are short-circuited.Thank you, bye.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 24, 2019, 07:19:46 pm
content: The pot is only setting the common mode voltage, so if would hardly effect the reading from the ADC. So no need to use a pot - just two fixed resistors are enough.The zero adjustment / compensation is better done digitally. So read zero with shorted inputs and subtract digitally.The TL082 is cheap, but not very good: it needs at least some 7-8 volts and does not work below some 3 V.A better cheap choice would be MCP6001 - it also works from the same 3-5 V supply as the ADC.

author: stcoso
author_rank: Regular Contributor
timestamp: March 24, 2019, 07:26:55 pm
content: Ok, thanks! In which respect (besides the input voltage) the TL081 is not good for this application ?

author: Gyro
author_rank: Super Contributor
timestamp: March 24, 2019, 07:30:03 pm
content: The LMC662 has averyhigh input impedance and typ. <5fA level input current. It's specified down to 5V and is cheap - it's a dual too, so you can play with the other half for mid rail buffering or whatever.EDIT: Do keep an eye on the datasheet input common mode range though.

author: David Hess
author_rank: Super Contributor
timestamp: March 24, 2019, 08:32:42 pm
content: A TL081 can be made to work but there are better parts. The LMC6081 is my old favorite (and a good choice for this) but I know there are better parts now.A major improvement can be made with any part by bootstrapping the operational amplifier's power pins off of it output so they track the input signal and the common mode errors are reduced and the input impedance is increased significantly.This is commonly done in high impedance buffers to raise the input impedance. Ignoring AC effects, the input resistance is the change in current for a change in voltage. The input bias current depends on the common mode voltage so if the common mode voltage does not change because the power pins are bootstrapped to the output, then the input bias current also does not change.

author: not1xor1
author_rank: Frequent Contributor
timestamp: March 25, 2019, 07:30:15 am
content: Quote from: stcoso on March 24, 2019, 07:26:55 pmlimited input-output voltage range, high offset and offset drift...modern parts also feature less supply current which might be useful in some cases

author: iMo
author_rank: Super Contributor
timestamp: March 25, 2019, 08:58:22 am
content: FYI -https://www.eevblog.com/forum/projects/simple-dc-afe-for-adc-chips-with-unipolar-diff-inputs/msg2120179/#msg2120179That is a bootstrapped opamp.When you replace the input chopper opamp in my schematics with the above recommended opamps and remove the input protection transistors you may get a highR input.

author: iMo
author_rank: Super Contributor
timestamp: March 25, 2019, 09:24:03 am
content: Below a simulation of a simple HighR AFE for ADS1115 (and others).Mind the serial resistance of the +/-1V input voltage source is 1T ohm.PS: the precise values or the tempco of the R6/R3 divider is not critical as the differential ADC input cancels the errors out.The R17=100Meg simulates the internal impedance of the ADC.Simulation only, use at your own risk

author: Kleinstein
author_rank: Super Contributor
timestamp: March 25, 2019, 04:22:42 pm
content: The 25 ohms resistors shown in the simulation from IMO are as shown not helping. If better tolerance to capacitive load is needed, it takes an additional resistor and capacitor as direct feedback. There may be no real need to buffer the virtual ground / negative ADC input.The choice of OPs depends on the needs, like cost, noise, bias, drift. Good choices are something like MCP6001 (low cost), LMC6482 (and a few similar ones) for low bias and OPA145 for low noise at still good bias.It may be a good idea to have a series resistor (e.g. > 100 K) at the input for protection - the OPs often include diodes by might need a current limit.

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2019, 01:33:31 am
content: Quote from: Kleinstein on March 25, 2019, 04:22:42 pmYou have to be a little careful in applications where precision is important with MOSFET and JFET input parts which can have low open loop gain and low common mode and power supply rejection. The LMC6482 is far from "precision".Another thing to check if it is provided is the input bias current versus common mode voltage. In some parts, this curve is very flat while in others, the input bias current can vary considerably indicating low input resistance but such a part may still be suitable over part of its input common mode range.

author: stcoso
author_rank: Regular Contributor
timestamp: March 26, 2019, 06:38:05 pm
content: Thank you all.I'm not an EE but you can surely inspire me to learn something new.