PGA281: Input floating issues

Hi Shivakant,

you must provide a current return path to GND for the input bias currents of PGA281. See section "Input Bias Current Return Path" and figure 47 on page 23 of datasheet.

Kai

Thanks for quick response.
Out of three in figure 47 of datasheet ,which i should use??
I used 1st one but still not stabilize system(getting offset 0.25V when input is floating). 47K ohm is slandered as i connect 54.7K ohm across + & - terminal with reference to ground

In my case sensors output impedance is 200 ohm,350 ohm & 350 ohm.

Hi Shivakant,

only the top picture in figure 47 makes sense in your application.

What do you expect from the PGA281 when not connecting any sensor to the inputs? That it behaves perfectly? This is impossible. The least you have to expect is an increase of output offset voltage.

If the error budget allows, you could decrease the two resistors to about 3k...10k.

Kai

Hi Kai,
I connected 10K ohm at INP & INN with respect to ground reference as shown in figure 47,But @ gain=128 of PGA281 getting offset of -2.5V @ floating input.

Hi Shivakant,

is the PGA281 damaged? Try a fresh one.

Kai

Hi Kai,
I am using 32 PGA281 for my 32 channels & i am getting same problem for differential signal input configuration.

Hi Shivakant,

all are showing exactly -2.5V at the +input and -input with 10k biasing resistors to GND?

Kai

No,Some are showing -2.5 Some are showing -0.8V,Some are showing 1.2V like that. & this is also varying time to time.
If response will be @-2.5V than no problem for us but it is varying that is the problem due to which i am not able to calibrate my channels.

Hi Shivakant,

and what do you get at the outputs of PGA381?

Kai

I am providing 2.5V VOCM but getting Voltages @ VON=10mV & VOP=2.51V ,similarly for others getting VON=3.10 & VOP=2.1V like that..
My stages are shown in schematic attached previously.

Hi Shivakant,

The PGA281 schematic shows three different types of ground connections on the schematic. The VSON is connected to "DGND" , most bypass capacitors and pull down resistors appear to be connected to "GND" and the capacitors on the RC filters are connected to "AGND".

Are these different ground connections joint/connected together somewhere on the PCB layout? Are the "GND", "AGND" and "DGND" connections at the same voltage potential? For example, are VSP and VSN supplies referred to the same ground potential than VSON?

Please show on the schematic where these different ground connections are joined/connected together.
Also, for clarity, please show the resistors you have on the floating inputs and which ground they are connected to.

Thank you,

Best Regards,
Luis

Hi Luis,

Thanks for reply.

Yes these three grounds ie DGND,AGND & GND are joined through ferrite bead.

 

@ input of PGA281,

Configuration of Filtering is as follows:

Load cell has differential out which connected to PGA281  differential input through above circuitry.

Hi Shivakant,

separating GND, DGND and AGND by ferrite beads is no good idea at all. Use the scheme shown in the EVM:

www.ti.com/.../pga281evm

Good catch, Luis!

Kai

Hi Kai,

thanks for reply.

In EVN, same(common) ground is used for Analog,Power & Digital ground.

In my case  as this PGA has digital interface, i did separation through ferride bead making reference common for all three ground.I avoided the digital ground noise interference to analog ground through ferride bead.

Hi Shivakant,

yes, in the EVM all grounds are directly connected together, because the PGA281 is intended and designed to work that way. If you separate the grounds by ferrite beads you isolate the grounds from each other at high frequencies. This can destabilize the PGA281 and cause oscillations.

Kai

Thanks Kai.

Is there any option to compensate the grounding problem as of now i have designed board & doing testing of it.

So is there any option to carry ahead with this board??

Hi Shivakant,

can you confirm that the ferrite beads are responsible for your problems at all? Have you made measurements with the ferrite beads shorted? Would this fix the problems?

Kai

Hi Luis,

Thanks for your fallow up.

Actually i am @ customer side due to which not able to test above.I will let you know within this week.