I'm using the Eval 4114 board with a STM32 nucleo64. So far I configured 8 differential channels for Vin plus the VRef and the temperature.
I use the 16bit rounding mode with the status byte following. On all of them I don't receive the expected result:
1. differential vin
On the analog inputs I've connected 8 potentiometers (10k) with with the voltage 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 V with a supply voltage of 1V.
When I read these differential channels (unipolar) I get a result that is too small, about 59% of what is expected. All channels behave the same. If I use a supply voltage on the voltage dividers of 2V then all values are the double.
in principal the adc seems to work, but for some reason there is a hugh gain error. The internal offset/gain calibration does not change this. If I use an additional gain in my SW of 1.68, then all values are within 3mV to the external voltages.
What could cause these identical gain errors?
2. Vref measurement
reading the Vref results in a code of 65535d in 16bit mode, that's what I would expect (full scale). Is this the correct behaviour?
here I don't use the correction factor and get 25V when using the formula of the data sheet. From this I would learn that the formula is correct and internal measurements are correct. This would imply that the gain errors are caused externally on the demo board.
3. temperature measurement:
Temperature = (Conversion Result/477 μV) − 273.15
what is the convertion result? is it in volts using the voltage conversion formula first or directly the data bits?
Either way, the result is completely wrong. There is no further explanation in the data sheet....
this is extremely confusing. It is stated on page 5:
AVDD = 3.0 V to 5.5 V, IOVDD = 2 V to 5.5 V, REF− = AVSS = 0 V, DGND = 0 V, VBIAS− = 0 V, REF+ = 2.5 V -> this must be the single supply operation as AVss = 0V , so AVDD is allowed to be 3.3V
in the table below on page 5 is stated:
Absolute Input Pin Voltage AVDD ≥ 4.75 V −20 +20V
AVDD = 3.0 V −12 +12V
I would interpret this as it is allowed and possible to use AVDD = 3.3V in single supply mode but the voltages will be restricted to +-12V.
Why then there is a paragraph on page 16 stating:
Single-Supply Operation (AVSS = DGND)
When the AD4114 is powered from a single supply connected to AVDD, the supply must be 5 V. In this configuration, AVSS and DGND can be shorted together on a single ground plane.
IOVDD can range from 2 V to 5.5 V in this unipolar input configuration.
I tried both, no difference. But the point is that 5V/3V = 1.66, which is my gain error factor as explained above. Is this just coincidence? I hoped changing to AVDD = 5V instead of 3V3 would solve the gain problem, but it doesn't.
Has anyone experienced similar problems ?
Thanks for your help.