LM385 Differential Configuration

[kaykan]

I have a problem with the circuit attached, in  the schematics you found the voltage values expected and real.

I tried with another op-amp (LM6462) and this work as expected, the problem is that this op-amp work up to 15V and I would like to use VCC=VDD=30V at the end (LM385 is up to 32V), in any case I don't understand how to select the good op-amp for the application.

From the real.png, seems that the op-amp don't work for low difference V+ - V-, my first design have VDD=VCC=19V so  I think that the issue is that on the V+ I get a value too near with op-amp VDD of the unity gain, but now there are over 5V, but still not working..

Have any suggestion? What's are the most important op-amp characteristics?

[Andreas]

Have any suggestion? What's are the most important op-amp characteristics?

It´s simply the wrong circuit and the wrong part(s).
The tolerances of the resistors are eating up precision.
And the OP-Amp cannot deliver current when pulling output near negative rail.
(You would have to use a negative supply).

Have a look at the inner circuit of  the LTC6102 datasheet.

http://cds.linear.com/docs/en/datasheet/6102fd.pdf

If you don´t want to use the LTC6102 you can replace the OP-Amp by a TL061 .. TL081 and the P-Channel FET by a small signal FET like the BSS84P. (The TL061 has a common mode range of typical to the positive rail so that the supply does not need to be higher than the 13.65V).
The ratio of the 2 resistors ROUT/RIN gives the amplification.

With best regards

Andreas

[kaykan]

Hi Andreas,

the resistors are 18K 0.1%, the tollerance is too low?

Probably the issue is for the proximity to the negative rail, I will try with a negative rail at -3V.

Thank you Andreas!

[Andreas]

the resistors are 18K 0.1%, the tollerance is too low?

4 resistors = 4 times 0.1% in worst case.
And this is only at room temperature directly after production.
There are up to 1 or 2 % soldering or ageing drift if using cheap resistors.

at 14 V * 0.4% = up to 56 mV error only due to common mode voltage supression error.
With 100mV measurement range there is not much precision left of your 0.1%

There is a reason why the LTC6102 does not use the simple differential amplifier circuit.

with best regards

Andreas