Original question:
Hi,
In need PGA with gain option 1, 2, 5, 10, 20, 50, 100, 200, 500 and 1000 and gain error within 0.05%. I am unable to get any PGAs (suggest appropriate part no). I got PGA 204 and PGA 207by cascading both i can get the required Gain. both are instrumentation amplifier I have the following clarification
1. Is it possible to connect 2 PGAIs in series.
2. what will be the effect on gain error, offset error and stability.
Regards,
Praveen Kumar M
Hi Praveen,
I'd suggest a combination of PGA207 as your front-end PGA with a PGA103 to give the additional gain range. That should support all your gain options.
1. It's absolutely possible to cascade multiple PGAs, though if you use a differential-input PGA as the second PGA, ensure its common-mode range is not exceeded in your application.
2. Offset, and drift are both two-sided distributions (and are uncorrelated between two individual PGAs). You can find the worst-case value for each of these by simply summing these error sources for each amplifier, but a more meaningful combination may be to combine the typical values using a root-sum-squared method, since both can shift independently of one another.
The only unusual problem in this is that offset voltage (and, by association, offset drift) are input-referred, so the overall offset and drift of the system has to be considered in light of the configured gains. For example, the typical input-referred offset of the combined PGAs would be:
keeping in mind that the offset voltage for each amplifier may change with gain. A similar calculation can be performed for offset drift.
Since gain error accumulates through the two PGAs as a product, there's not an easy way to estimate a typical shift for it, however the worst-case should be fairly straightforward - the overall gain error will be (1 - (1 + Ge1)*(1+Ge2)) where Ge1 and Ge2 are the gain errors of PGA 1 and PGA 2 respectively..
Hi Praveen,
My apologies. Not sure why the image didn't attach properly before:
You can cascade PGA204 and PGA207, simply ground the (-) input of the second amplifier in the chain and tie the output of the first amplifier to the second's (+) input.
