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Part Number: LMP7721
Are there known issues operating this chip with +/- 2.5V supplies instead of a single rail 5V supply? I assumed it would work with dual rails provided the total differential voltage was within limits, but now I'm not so certain.
My supplies come from a LM27762, and I am seeing a lot of noise on the output of the LMP7721 that has been amplified from the supply rails. The PSRR spec of 92dB is specified for a single rail supply, and it has made me question if attempting to use dual rails is the source of the trouble. I'm using the LMP7721 as a current integrating preamp in the following configuration, so the gain is fairly substantial:
Low Power Amplifiers and Comparators (LPAC) Applications
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In reply to Paul Grohe:
Thanks, I'll give that a shot. The load should be minimal, as it only feeds subsequent gain stages. Is the noise via the ESD diodes something unique to this part? A previous version of this design made use of an IVC102 running off of +/- 15V supplies with worse PS noise, and almost nothing coupled through the pre-amp. I'll keep an eye on how much the supply resistors drop the voltage at the chip. Full output range isn't super critical in this stage, but the closer I can get to it the better.
In reply to Adam Shrey:
It is common for CMOS op-amps to have ESD protection, due to the much lower gate breakdown voltages.
Below is the common ESD protection scheme for CMOS amps. Pretend each diode is also a 0.3pF to 0.5pF cap and you can see the paths. For reference, the Xc of 0.5pF at 20kHz is 8 Megohms....
Older JFET based devices may not have these ESD diodes ,as the older high voltage JFET/Biolar devcies tend to be a bit more "rugged".
I am not familiar with the IVC102 internals, but at 30V, it is most likely a JFET-based device and may not have these ESD input diodes.
So for very low current measurements, you really want to keep the supplies clean, possibly dedicating a separate filtered supply for the front end amplifiers (using R, C and even L's and ferrite beads). Also watch out for power supply traces running under signal paths.
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