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SMU — Source Measure Unit

Auto-generated reference

For the definitive command list extracted directly from source code, see REPL Command Reference.

Controls source measure units (SMUs) that can both source a precise voltage and simultaneously measure current (or vice versa).

  • Multiple SMUs are named smu1, smu2, etc.
  • Address a specific SMU directly: smu1 set 5.0 — or use use smu1 then smu set 5.0

±60 V / 3 A four-quadrant SMU (20 W source, 12 W sink). Uses the nidcpower Python package (not VISA/SCPI). Install with: pip install nidcpower. Output range: −60 V to +60 V, −3 A to +3 A DC (10 A pulse).

PXIe boot order

The PXIe chassis must be powered on before the host PC boots. PXIe devices are enumerated during BIOS/POST — if the chassis powers on after the PC, the instrument will not appear in scan or list. Power cycle the PC with the chassis already on to fix this. See Troubleshooting for details.

smu on / smu off

Enable or disable the SMU output.

smu on     # enable output
smu off    # disable output

Safety limits set with upper_limit / lower_limit are enforced before enabling.

smu set

Set output voltage and optional current limit.

smu set <voltage> [current_limit]
Parameter Required Values Description
voltage required −60.0 – 60.0 (V) Target output voltage.
current_limit optional 0.0 – 3.0 (A) Current compliance limit (always positive). If omitted, the existing limit is kept. 
smu set 5.0          # set 5 V, keep existing current limit
smu set 5.0 0.01     # set 5 V with 10 mA current limit
smu set -12.0 0.1    # negative voltage for four-quadrant operation

Warning

Setting a voltage without a current limit uses whatever limit was set previously (default: 10 mA). Always specify the limit when powering an unknown DUT. DC source power is capped at 20 W and sink power at 12 W by the hardware.

How to sink current

Use smu set_mode current instead of smu set. In current mode, the SMU acts as a programmable electronic load — positive current = sink from DUT, negative current = source into DUT. See smu set_mode below.

smu meas

Take a single measurement and print the result.

smu meas [v|i|vi]
Mode Alias Description
(no arg) or vi both, all Atomic V+I+compliance read in one call (default)
v volt, voltage Measure actual output voltage only
i curr, current Measure actual output current only 
smu meas       # prints e.g.  V=5.000012V  I=0.009987A
smu meas vi    # same as above; appends [COMPLIANCE] if current-limited
smu meas v     # prints e.g.  5.000012V
smu meas i     # prints e.g.  0.009987A

Tip

smu meas / smu meas vi performs an atomic measurement that reads voltage, current, and compliance state in a single driver call — use this instead of separate v and i calls when you need a consistent snapshot.

smu set_mode

Switch the SMU between voltage-source and current-source modes.

smu set_mode voltage <v> [current_limit]
smu set_mode current <i> [voltage_limit]
Parameter Required Values Description
voltage / current required mode keyword Select sourcing mode. Alias: v / i.
second arg required float Setpoint in volts (voltage mode) or amps (current mode, −3.0 to 3.0).
third arg optional float Compliance limit for the opposing quantity (always positive). 
smu set_mode voltage 3.3 0.1    # source 3.3 V with 100 mA current limit
smu set_mode current 0.010 3.0  # sink 10 mA from DUT (electronic load), 3 V compliance
smu set_mode current -0.5 5.0   # source 0.5 A into DUT, 5 V compliance

Current mode polarity

In current mode, positive current = sink (draw current from the DUT, acting as a load) and negative current = source (push current into the DUT). This is the standard four-quadrant SMU convention. Use positive values when the SMU is loading an LDO, regulator, or other voltage source.

Safety limits (upper_limit / lower_limit) are enforced before the mode switch is applied.

Recording measurements

Use assignment syntax to measure and store the result to the measurement log in one step:

<label> = smu meas <v|i> [unit=<str>]
Parameter Required Values Description
label required string, no spaces Name for this entry in the log.
v\|i required v, i What to measure.
unit= optional string Unit shown in log print. Defaults to V or A. 
smu_vout = smu meas v unit=V
smu_iout = smu meas i unit=A
calc power smu_vout * smu_iout unit=W

See Log & Calc for full details.

smu compliance

Query whether the SMU is currently in compliance (i.e. the output is being current- or voltage-limited by the instrument's protection circuit).

smu compliance

Prints Not in compliance when the DUT is drawing within limits, or a warning banner IN COMPLIANCE - output is current-limited when the limit has been hit. Use smu meas vi to see the compliance flag alongside a measurement.

smu source_delay

Get or set the settling delay the SMU inserts between applying a new source value and taking a measurement.

smu source_delay           # read current delay
smu source_delay <seconds> # set delay (0 – 167 s)

smu source_delay           # prints e.g.  source_delay = 0.0020 s
smu source_delay 0.05      # set 50 ms settle time
smu source_delay 0         # disable delay

smu avg

Get or set the number of samples averaged per measurement.

smu avg        # read current value
smu avg <N>    # set to N samples (integer ≥ 1)

smu avg        # prints e.g.  samples_to_average = 1
smu avg 10     # average 10 readings per measurement call
smu avg 1      # back to single-sample mode

Higher values reduce noise but increase measurement time proportionally.

smu temp

Read the internal temperature sensor of the instrument.

smu temp

Prints the temperature in degrees Celsius, e.g. 34.5 degrees C. Useful for confirming the unit has warmed up to a stable operating temperature before precision measurements. Expected range for a powered-on PXIe-4139 is approximately 20 – 60 °C.

smu get

Show the current voltage setpoint, current limit, and output state.

smu get

Prints: Setpoint: <V>V @ <A>A, Output: ON|OFF

smu state

smu state <on|off|safe|reset>
Value Effect
on Enable output
off Disable output
safe Disable output
reset Disconnect and re-initialize the session

Safety limits

upper_limit and lower_limit work with SMUs using the same voltage and current parameters as PSUs:

upper_limit smu voltage 5.5     # SMU output ≤ 5.5 V
upper_limit smu current 0.1     # current limit ≤ 100 mA
lower_limit smu voltage -0.3    # SMU output ≥ −0.3 V

See Safety Limits for the full reference.

Typical workflow

upper_limit smu1 voltage 6.0
upper_limit smu1 current 0.05

smu1 set 5.0 0.02     # 5 V, 20 mA limit
smu1 on
sleep 0.5

vout = smu1 meas v unit=V
iout = smu1 meas i unit=A
calc power vout * iout unit=W

log print
smu1 off