about ads1291 pga setting, whether it is linear or db?

Mingling Hsu

Other Parts Discussed in Thread: ADS1291

hi all:

I am making a gadget with ads1291, I happened to found the pga setting is not that i used to think of.

As many function described in the datasheet use the pga setting directly. such as Vref/pga. From this point, it seems to be linear. however this is what i found:

Here the pga setting does not look like either linear or db. what's the problem?

over 9 years ago

0 Tom Hendrick over 9 years ago

TI__Guru**** 180931 points

Hi Mingling,

It is a linear function. Tell us more about how you have the ADS1291 configured (register settings, voltage sources, reference etc).

0 Mingling Hsu over 9 years ago in reply to Tom Hendrick

Prodigy 110 points

hi Tom,

Good to hear from you!

I use internal reference(4.033v), 3.3v power supply (analog & digital),

following is my regs dump:

0: 0x00000052

1: 0x00000006

2: 0x000000b0

3: 0x00000010

4: 0x00000010

5: 0x00000000

6: 0x00000000

7: 0x00000000

8: 0x00000000

9: 0x00000000

10: 0x00000002

11: 0x00000002

shcematic is the same as my previous one as followed pic. As you mentioned last time, the dgnd/agnd was connect immediately beneath 1291.

Thanks for your heal

0 Tom Hendrick over 9 years ago in reply to Mingling Hsu

TI__Guru**** 180931 points

Hi Mingling,

I don't see any bypass caps or caps between VREFp and VREFn, which could be part of your problem. Is the chip on your board the same chip you were using when you posted the issue about your bi-polar supply?

0 Mingling Hsu over 9 years ago in reply to Tom Hendrick

Prodigy 110 points

hi Tom,

sorry for the incomplete of the schematic . I have caps between vrefp and gnd, and vrefn tie to ground, pls refer to the followed pic. could that be a problem?

It's not the same board as my previous post about bi-polar supply(I give up that problem for now).

0 Mingling Hsu over 9 years ago in reply to Tom Hendrick

Prodigy 110 points

hi Tom,

I've found something new. When I test internal test signal, I found the following:

It seems that the pga in 1291 did not amplify singular end potential by linear, but rather amplify signal from INxP to INxN by linear. Is that suggesting that i can't trust the result with pga>1 , if I use single end input, where signal feed to IN1P and gound was tied to IN1N?

0 Tom Hendrick over 9 years ago in reply to Mingling Hsu

TI__Guru**** 180931 points

Hi Mingling,

When driving a single ended input to the ADS1291, it's best to tie IN- to a common mode voltage. Please refer to the 'ANALOG INPUT' section of the datasheet which begins on page 20. Please also refer to Figures 21 and 22.

0 Mingling Hsu over 9 years ago in reply to Tom Hendrick

Prodigy 110 points

Hi Tom,

Test & verified， not using common mode above gnd as INN input is the cause of the problem .Thanks.

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about ads1291 pga setting, whether it is linear or db?