I'm using an AD620 which is currently running on +/- 5V supplies. I can simplify my power system by using +6V and -5V will this affect the
performance of the A/D or its output values ?
Some more detail about your circuit would help us come to a better solution.
I have a couple more things to consider:
If I understand correctly, it sounds like you are trying to maximize your ADC dynamic range by being clever with the supplies. Sometimes this works if you're careful. Harry nailed down the two main considerations. The PSRR issue requires careful attention to decoupling, but the extra 1V dc can cause about 100uV RTO extra offset, which can degrade a bit at higher gains, as shown in the specifications. This part of it is a static offset error if you calibrate. The bigger problem is ADC analog input protection. Harry pointed out that you could go outside the range on the top side. I would be more concerned about the low side. If the ADC input is 0-5V, then a negative voltage will turn on the ESD diodes of the ADC. A -4V output from your amplifier is going to need a larger protection resistor to limit the current into the ADC inputs. And I wouldn't suggest relying on the short-circuit current to take care of the over-voltage. It's not a well-controlled number in amplifiers, it generates a lot of heat, and it could slowly wear away the metal in the ADC input until you get failures later on.
One more thing. Be careful of the hexagon plot behavior (the internal nodes of the instrumentation amplifier can be railed by a combination of common-mode, gained differential, and Vref). This web tool would help with the 620. http://designtools.analog.com/dt/inamp/inamp.html?inamp=AD620
Our newer in-amps have plots that show this limitation published in the data sheet.
It might be easier to use a rail-to-rail output in-amp and run it on the same 5V supply as the ADC. Something like the AD8226 would be good for this. Especially if you can control your input common-mode voltage.
I hope this helps. Let us know if you have any more questions.
If I understand your question, you have an AD620 InAmp driving an ADC or possibly a filter and
then into the ADC. You may want to post a partial schematic so we can address the details, but
some general things to consider.
1) The PSRR from the V+ terminal, and the PSRR from the V- terminal are a function of frequency.
See figs 15 and 16 on the datasheet. If the 6V is from a 100kHz switching regulator and has noise
on it, you are in trouble.
2) If the ADC runs on ground and 5V, then when using +/-5V, there is no way you can put more than
+5V into the input of the ADC. If the AD620 runs on +6V/-5V, and you have a huge input signal that
causes the output of the AD620 to slam to the top rail, you now have a high voltage into ADC and that
might turn on the ESD network and blow up your ADC. However, the AD620 output is not rail-to-rail,
so it won't go to 6V. However, the spec table guarantees at least (Vs+)-1.2V = 4.8V. However,
that is a MINIMUM, so a good part with a beefy output might give you 5.1V or 5.3V or whatever.
The AD620 is one of the grandaddies of the InAmp world. We have quite a few newer parts on
newer processes that are less money, so if you are in the prototyping stage, you may want to
consider some other part number. I'm not in the InAmp group, so I'll defer to those guys to
suggest some other numbers to look at.
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