AnsweredAssumed Answered

AD7682 severely influences input signal

Question asked by UwaertsB on Oct 29, 2015
Latest reply on Nov 6, 2015 by ngray

Hello,

 

In a current design for a test system, we use a power supply circuit with variable current limit and voltage setting. This circuit is controlled with DAC's and measured with ADC's (AD7949BCPZ).

Attatched picture "PSU circuit.jpg" shows the circuit. In this circuit, the ports 'PSU_Imeas' and 'PSU_Vmeas' are connected to ADC inputs. This circuit operates exactly as intended.

When operating the ADC, it is impossible to achieve accurate readings. After investigation we notice a strange phenomenom: when the ADC acquires data, the signal it is supposed to measure is pulled to ground, followed by heavy oscillations.

The effect is more pronounced at PSU_Imeas, but nonetheless present at the PSU_Vmeas node.
Attached to this post you can find a screenshot of what we see on the oscilloscope, where the yellow signal is the ADC CONV signal and blue is the PSU_Imeas node.

We tried replacing the ADC with different models in the same family: the AD7682 and AD7689. Both gave similar results.

We checked the opamp datasheet, it should not have any problem driving the load.

 

Has anyone seen this happen in their project? Any idea what it might be caused by?

Attachments

Outcomes