Electronics > Beginners
(Very) High Impedance Voltmeter and ADS1115
(1/3) > >>
stcoso:
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 +- 0.5V

I 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.
Kleinstein:
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.
stcoso:
Ok, thanks! In which respect (besides the input voltage) the TL081 is not good for this application  ?
Gyro:
The LMC662 has a very high 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.
David Hess:
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.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod