Other Parts Discussed in Thread: ADC3140EVM-PDK
We have a TLV320ADC3140 running on 3V3. AREG is measured to 1.8V and DREG is measured to 1.5V.
The TLV320ADC3140 is configured to MICBIAS=VREF=2.75V. 2u2 on VREF. But I measure it to 2.4V. There is a 20kohm load on MICBIAS.
If I change VREF to 2.5V, I measure it to 2.2V. If I change VREF to 1.375, I measure it to 1.2V.
I cannot figure out why. Please help.
A related question is the input range. The inputs are configured to DC-coupled and single-ended input. All gains at 0dB. No filters enabled. Using TDM 32-bit mode.
It seems that the valid input voltage range is from apx 0V to VREF/2 corresponding to digital output values from 0x0000 0000 to 0x7FFF FFFF. In pracis a bit less.
I was expecting a range from 0V to VREF with a digital output from 0 to 0xFFFF FFFF or alternatively -0x8000 0000 to 0x7FFF FFFF. But apparently not. But why.
E.g. a AC signal on 2.75V peak-peak with a DC bias at 1.375V to give approximately the largest possible signal to convert. I know that in practice the rails are not perfect.
It seems that either there is a gain on +6dB on the analogue input on chip, or alternatively the inputs voltage range is from -VREF/2 to +VREF/2. (I now that the latter is not very likely as there is no negative supply on chip, and the absolute maximum ratings specify the inputs not to drop below -0.3V)
The input range and the output values does not change if I change the configuration to differential mode. (INx- is grounded).
These observations does not match with the fact that the chip supports 2VRMS in differential mode and 1VRMS in single-ended mode (When using VREF=2.75V). But only the half.