LTC2057 output offset

[Bgdn]

Hello,

i have made the circuit from the picture bellow.

The input voltage comes from ad8221,  and the output voltage is feeding a adc ( ltc2368-24).

The power supply for V+ is 3.3V, and for V- is -2V.

Input signal is DC.


My problem is that i have a large amount of offset voltageoutput of LTC2057 which is aprox 51mV

I measured 1.4415 volts at the entrance ( V3 in the schematic), and on the output i get 1.3828Volts.

Why is this offset? I am missing something here?

[Kleinstein]

The output load may be too capacitive - so too little series resistance to isolate it from the input. With the low impedance output the layout (especially the ground) can be a factor too.
So there is a chance the OP-amp oscillates or at least has strong ringing after the internal switching spikes and spikes coming from the ADC.

At least for a test one could increase the 10 Ohm to let's say 100 ohm.

The supply is rather low, but should normally be OK. With only some 5 V there are more alternative to the LTC2057, that may have better tolerance to capacitive loading.

[Dr. Frank]

The output might oscillate very probably, please check.
Remove the 4n7 capacitor from the output.
Frank

[dietert1]

The schematic does not show buffer caps on the supplies. Sometimes buffer caps help with capacitive loads.

Regards, Dieter

[Bgdn]

Hello,

the power voltage rails are buffered with 100n & 10uf caps.

The pcb is a 4 layer one,  with a solid ground layer, with no loops on the other layers

I know the adc sar, has some spikes durring aquisition.


I will try tonight to do a debug with a scope on the lines to see if i have some oscilations.

I will keep you updated.

[Bgdn]

Hello,
i have mounted the scope at the entrance of the adc,  there was an oscilation of about 50mv ( unfortunatly i did not found the stick to export the scope's picture).

I removed the 4n7 capacitor, and the oscilation disapeard.

Everything seems to work fine now.

By the way, can you recomend some low ripple power supplies for negative/positive rails?

In my design i use XC6210B332MR for the 3v3 rail, and LTC1550LCMS8-2 for the negative voltage.

[MiDi]

By the way, can you recomend some low ripple power supplies for negative/positive rails?
In my design i use XC6210B332MR for the 3v3 rail, and LTC1550LCMS8-2 for the negative voltage.

LT3045/3094

[Andree Henkel]

10 Ohms is indeed at the low end, values for the isolation resistor are 10..100

circuit ca be improved with double feedback:
problem with starting circuit is, that ADC mesurement value will change with sample rate: rising sample rate -> drop in result value
this ca be cured with feedback from the reservoir capacitor instead of OP-Output, but with a resistor, typically 1k
but now you also need a feedback capacitor from OP output, typically about/less 1/100 of reservoir capacitor value.