ADS4225: Full scale input swing vs gain

Emmanouil Tsachalidis

Part Number: ADS4225

Hi,

I am a bit confused as to what the full scale differential input swing of the ADC is. The datasheet clearly states that it is 2Vpp when ADC programmable gain is 0dB and 1Vpp when gain is 6dB which makes sense.

Now the supply rail is recommended to be at 1.8V (single). Is it normal for the ADC input swing to be that high given the supply rail at 1.8V?

Is the differential input swing related to the AVDD?

For example, if the AVDD is 1.8V then the digital outputs will send 4095 when the differential input swing is 1.8Vpp, if the AVDD is 1.9V then the digital outputs will send 4095 when the differential input swing is 1.9Vpp and so on as long as absolute maximum ratings are not violated.

Is that what it is?

Pls confirm

Best regards

Manos Tsachalidis

over 5 years ago

0 jim s over 5 years ago

TI__Guru* 91947 points

Manos,

The full scale of the ADC is not tied to the supply voltage but rather based on an internal bandgap reference which is independent of the supply voltage.

2Vpp means the input signal is VCM +/- 500mV on each of the two input pins. VCM in this case is 0.95V thus the input voltage levels on the differential input pins are 0.95V +/- 0.5V = 0.45 to 1.45V.

When digital gain is used, the maximum input full scale is reduced. Here are the max codes in offset binary format for different full scale voltages for 2 different digital gain settings. This is independent of power supply.

Input Fullscale	0dB Digital Gain	6 dB Digital Gain
2 Vpp	4095	n/a
1 Vpp	2047	4095
0.5 Vpp	1023	2047

Regards,

Jim

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ADS4225: Full scale input swing vs gain