Other Parts Discussed in Thread: ADS1258, OPA735, OPA734
Hi Friends.
I'm using the ADS1258 for data acquisition of bipolar channels (each one ranges from +2V5 to -2V5 and are already buffered by opamps).
The AD voltage reference pins were mean to be +2V5 and -2V5.
On our tests we are seeing too much noise on the AD measurements. Inspecting the hardware we noted that our negative reference is oscillating a lot. We see a charge/discharge ramp over the capacitor C51 shown on the picture below.
The positive reference is fine, with noise below 10uV. The problem is the negative reference.
Getting the C51 out makes the negative output to be stable again, but the noise is prohibitive, around 10mV. I already used a topology like that for the reference with no problem at all, but now seems that the negative reference is driving too much current from the capacitor.
The reference circuit is depicted below. The REF2V5 output is connected to VREFP and the REFN2V5 is connected to VREFN of the ADS1258.
Any hints or suggestions about making good +-2V5 references for the ADS1258?
I am planning on using a 5V high precision regulator (tied to a negative supply of -2V5 and a positive supply of +5V) to generate a stable and reliable positive 2V5.
As it will be referenced to the -2V5, the differential voltage would be fine. Some offset would arise between the center of the reference and the ground, however it can easily compensated by software or using chopping.
Any suggestion is very well appreciated.
Thanks again for the attention.
Eduardo Pellini
