Question about constant current mode on benchtop power supply

[_Indy_]

Not new to electronics but very new to using a benchtop power supply.

I'm using a Rigol DP832 (with the high precision option) and am trying to learn the basics.  I attached the leads to channel 1 and connected a 100 ohm (10W) resistor.  I set the voltage to 10V and the current to 50mA.

I expected the current to be limited to 50mA and for the channel to be in CC mode.  It did go into CC mode but the output current shows 56 or 57mA.  As I move the current setting up or down the output current shown is always 6 or 7 mA higher.  I tried the same thing on channel 2 and seems to behave a littler closer to what I expected - the output current is only 1 or 2 mA higher than the setting.

A screenshot of the two channels is attached.

Am I misunderstanding:
- how constant current mode works?
- the accuracy of the output current shown?
- the accuracy of the power supply in general?

Any insight or education would be appreciated.  Thanks.

[Kim Christensen]

Looks about right to me as per the specs:

[ArdWar]

Your understanding is correct, although you may be a bit too optimistic with your accuracy expectation. It isn't uncommon for DC supplies to have a couple of mA/mV setpoint offset.

However 7mA at 50mA is unexpectedly large and according to their datasheet technically just about outside the spec. Assuming that the error is entirely at setpoint/output. Readback error is usually much smaller, and if your 100Ω load is accurate, the voltage readback support the current readback.
Rigol publish calibration manual if you're willing to twiddle with it (https://beyondmeasure.rigoltech.com/acton/attachment/1579/f-03a3/1/-/-/-/-/DP800%20Calibration%20Guide.pdf)

By the way, high resolution option (the A version) does exactly what it says on the tin. It only increases resolution and afaik does nothing with precision nor accuracy.

[_Indy_]

Looks about right to me as per the specs:

Thank you for the information.

Your understanding is correct, although you may be a bit too optimistic with your accuracy expectation.

Yes, I was probably too optimistic.

An external measurement of the current closely matched the readback (displayed) current so the error/offset would seem to be on the setpoint (output) side.

After I become more familiar and experienced I might take a look at the calibration manual.  Thank you for your help.

[RoGeorge]

To have mV and mA accuracy, DP832 will need a recalibration after upgrade.  There are 4 convertors to calibrate for each channel (12 in total)
- the set voltage DAC
- the set current DAC
- the read voltage ADC
- the read current ADC

All 4 has to be calibrated for 1mV or 1mA accuracy.  To do that, another instrument is needed, to measure the generated voltages (with no load), and the generated currents (with short circuit), and a calibration script to automate the calibration process.  In theory it is possible to calibrate manually, in practice that will take forever.

I've calibrated my DP832 using the script from:
https://www.eevblog.com/forum/testgear/dp821a-calibration/msg2840794/#msg2840794
https://www.eevblog.com/forum/testgear/rigol-dp832a-automatic-scpi-calibration-script/
after changing it such that the script can read the V and I measured from a 5 digits mtx3283 DMM (using MODBUS over serial).