Negative voltage to Ext common mode pin for slow ADC

[priit]

I'm using STEMlab 125-14 4-Input. I want to measure +- 1.5 V values with the slow ADC. I removed, the 0 Ohm resistor C143 connecting the negative inputs to the ground as shown in https://downloads.redpitaya.com/doc/Red ... N_V1r3.PDF. I provided a negative -1.5 V to ext. com mode (E2 pin 11), and +1 V to Analog Input 0 (E2 pin 13) and expected to measure around 2.5 V, however, the readings were nonsense (around 0.18 V). When I reconnected the pin 11 to ground, the readings were as expected. Is it possible to measure voltages using a negative common mode voltage on slow analog inputs and how to do it?

[redpitaya]

Hello priit,

The described procedure should work.

From the software side, the slow analog inputs can measure voltages from 0 to 3.5 Volts. If a negative voltage is applied to the ext. com. mode pin, then you should be able to offset the pins (if the resistor C143 is removed). There might be a bug in the software. Try measuring the RAW values and see whether they change properly.
Try connecting the GND pin of your negative voltage source to the Red Pitaya to synchronise the grounds of both devices. That might help.

Otherwise, please let me know, and I will inform the developers.

[priit]

Hi, thanks for taking the time. I re-did the test with both negative and positive reference voltages on ext. com mode pin. It seems that analog inputs work with positive reference, but not with negative.
I had the setup as you suggested:
* C143 removed
* RP GND connected to voltage supply 0
* V_ref connected to ext. com mode and ranging from -0.7 to +1.1 V from GND.
* +1V (from GND) applied to A0 (Analog Input 0 E2 pin 13)
* V_exp. is the expected voltage value (+1-Vref) confirmed by multimeter.

The results are as follows and also plotted here. The minimum working reference voltage is around -0.1 V. So there seems to be a problem with negative reference voltages on ext. com mode pin.

Code: Select all

					

		|Measured
V_ref	|V_exp.	|V 	Raw
-0.7	1.7	0.28	161
-0.5	1.5	0.49	285
-0.3	1.3	1.14	669
-0.1	1.1	1.09	638
0	1	0.99	588
0.1	0.9	0.9	526
0.3	0.7	0.7	412
0.5	0.5	0.5	292
0.7	0.3	0.29	172
1	0	0	2
1.1	-0.1	0	0

I also have a STEMlab 125-10, I could test also on that one, but I can't find a schematic that would indicate which one is the resistor R131 to remove?

[priit]

Hi, I actually found a solution to my original problem which was measuring bipolar voltages on slow analog inputs. I went throught the Zynq XC7Z020 manual and realised the slow analog inputs can be switched to bipolar mode. So I went through the v0.94 fpga project and sure enough, in the block diagram XADC wizard has a setting in the Channel Sequencer page to switch the ADCs to bipolar mode. After rebuilding the fpga I can now read slow analog inputs in the +- 3.5 V range. They are read as 12 bit two's complement values.

[redpitaya]

Hey,
Thank you for taking the time to test this . I will update the docs to include your findings so that other users are aware of this.

If you need the resistor reference for the STEMlab 125-10, I can check the schematics and let you know.

[priit]

Yeah, my pleasure.

If you need the resistor reference for the STEMlab 125-10, I can check the schematics and let you know.

Yes, please, it would be good to know the location of the R131 resistor on the STEMlab 125-10.

[redpitaya]

The 125-10 schematics are not together with the other schematics. I asked the team to see if they have them somewhere else. I'll let you know once I get them.