Tool/software:
Hello Support Team
I plan to use an LM27762 for the bipolar power supply of the INA851 instrumental amplifier in my design. However, due to the low load current, the LM27762 will switch to PFM mode. Will this ripple cause problems for the INA851 outputs?
Hello Bekir,
To understand the effect that power supply ripple will have on the device, we must look at the PSRR of the INA851. At low frequency, a sinusoidal change in power supply will result in an input offset error that is attenuated by 110dB. This means that for a 1V, 10Hz sinusoid on the power supply, this means you will have a 3.16uV of offset error on the input. This will result in a different amplitude output error based upon your gain.
If the ripple on the supply rail is not a sinusoid, running an FFT on the noise and seeing the amplitudes of each frequency component of the ripple will help determine how much input referred error you can see.
Apparent PSRR can be increased by adding filtering between the LM27762 and the supply pins of the INA851. The reason I say apparent PSRR, is that the input filtering will mean that the actual ripple voltage seen at the pin of the device will be much smaller, therefore the total power supply rejection will be increased, but the actual power supply rejection of the instrumentation amplifier is unchanged.
Best,
Gerasimos
