LMP7721: LMP7721 rails to V+ when a Piezo crystal is connected

Manjunath Prabhakar1

Part Number: LMP7721

Hi,

The LMP7721 eval kit is set to a gain of 33, V+ is 2.5V and V- is -2.5V. Vref is ground. Non-inverting default mode. When a 1mV square signal is connected from a signal generator, I get the required output gain. But when I connect a Piezo crystal the Output is railed to +2.5V. Only when I excite the crystal which should produce a good enough signal, I see some small negative bumps in the oscilloscope. Please let me know what changes are required to get the amplifier working for a Piezo crystal.

Regards

Manju

over 4 years ago

0 Marek Lis over 4 years ago

TI__Guru 60590 points

Manju,

It would be really helpful to see your schematic. Having said that, I suspect you do not have a path for IB current to flow. You must provide an input bias path to ground (see Rbias below) for LMP7721 or circuit will not work properly - Piezo Crystal is not capable of driving the input and thus should be AC coupled, C1 - see below.

LMP7721 Piezo Crystal.TSC

0 Manjunath Prabhakar1 over 4 years ago in reply to Marek Lis

Prodigy 90 points

Thanks Marek, the amplifier is working with Piezo crystal once the circuit was modified as you suggested.

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LMP7721: LMP7721 rails to V+ when a Piezo crystal is connected