Source Measure Unit
Compliance Limits (Clamps)

How can an SMU limit the Voltage or Current?

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How does the hardware protect against damage to the Device-Under-Test (DUT) and the Source Measure Unit (SMU) itself? Two extreme conditions are forcing voltage into a SHORT or forcing current into an OPEN. The compliance circuit provides two limiting or clamping actions

When Forcing Voltage, Clamp the Current.
When Forcing Current, Clamp the Voltage.

You can set these Compliance Limits based on a maximum operating limits of the device or the SMU.

But how can we add these limits to the basic SMU circuit? By simply adding two error amplifiers to the servo loop, we can achieve clamping action. The final challenge is creating a Crossover circuit that seamlessly transfers control from Force Mode to Clamp Mode. (See Crossover Circuit topic.)

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OVERVIEW

To expand our SMU's capacity to implement clamps, we add blocks (shaded in ORG) for the POS and NEG Clamps.

Each of the three control modes - Force, POS Clamp and NEG Clamp - requires its own Error Amplifier.
The Crossover Circuit pass through the MINIMUM of the three errors.
The Clamp Feedback selects the opposite signal of the Force Feedback.

FORCE VOLTAGE, CLAMP CURRENT

As an example, suppose you want to force +5V with clamps set at +1mA and -1mA.

Set Points

set = +5V
set_clp = +1mA
set_cln = -1mA

Feedback

fb = MV (voltage)
fb_cl = MI (current)

FORCE CURRENT, CLAMP VOLTAGE

Alternatively, suppose you want to force 1mA with clamps set at +5V and -5V.

Set Points

set = 1mA
set_clp = +5V
set_cln = -5V

Feedback

fb = MI (current)
fb_cl = MI (voltage)

CLAMP CIRCUIT

The Error Amps for Clamps (U6, U7) look similar to one for Force (U1).

The set points for POS and NEG CLAMP (typically DACs) are set by V_SETCLP and VSETCLN.
The Clamp Feedback fb_cl is fed to both POS and NEG error amps.
Error signals err_clp and err_cln measure the error between the clamp feedback and the set points.
The Crossover is a clever circuit that passes through the MINIMUM of the three errors: err, err_clp and err_cln.
(See Crossover Circuit topic!)

Feedback selected by switches S1 - S4 controlled by fmode (0V=FV, 5V=FI).

Force VOLTAGE, Clamp CURRENT
- S1 ON: fb = MV
- S4 ON: fb_cl = MI

Force CURRENT, Clamp VOLTAGE
- S2 ON: fb = MI
- S3 ON: fb_cl = MV

SMU SPECS

Here's a quick overview of the operating ranges of the SMU circuit:

Vrange = +/-5V,
Irange = +/-10mA
Current Sense
- Rs = 100
- Vs = +/-1V Max
Internal voltage range for feedback loop.
- Vset = +/-5V (5V or 10mA Range)

SIMULATION

Enable the relevant PARAM before running each test.

SPICE Netlist: Remove the "*" before the PARAM statement. All others add "*" at beginning of statement.
LTSPICE Schematic: Right-click the statement and select DIRECTIVE. All others select COMMENT.

FORCE V, CLAMP I Set the Force Voltage parameters (.param fmode=0V...) as a SPICE Directive. Set Force Current (.param fmode=5V...) as a Comment.

.param fmode=0V set=1V set_cl=1mA/10mA*5V Rs=100 Rdut=1000

Force Voltage: 1V
Clamp Current: 1mA on the 10mA Scale
Sense Resistor: 100 ohms
Device-Under-Test: Rdut = 2000
Output Current: Idut = 1V / 2000 = 0.5mA

Run a Transient Simulation of SMU-circuit-clamps-1.cir (or *.asc). Add traces for output voltage v(vo) and current I(Rdut). Add another Plot Window and Add trace I(Rdut). Does Idut reach it's expected value of 0.5mA?

Now reduce Rdut to 500 ohms for an expected output current of 2mA. Rerun the Transient Simulation. Did the POS Clamp swing into action and limit the output current to 1mA?

FORCE I, CLAMP V Set the Force Current parameters (.param fmode=5V...) as a SPICE Directive. Set Force Voltage (.param fmode=0V...) as a Comment.

.param fmode=5V set=1mA/10mA*5V set_cl=2V Rs=100 Rdut=1000

Force Current: 1mA on the 10mA Scale
Clamp Current: 5V
Sense Resistor: 100 ohms
Device-Under-Test: Rdut = 1k
Output Voltage: Vdut = 1mA * 1k = 1V

Run a Transient Simulation of SMU-circuit-clamps-1.cir (or *.asc). Add traces for output voltage v(vo) and current I(Rdut). Add another Plot Window and Add trace I(Rdut). Does Idut reach it's expected value of 1mA?

Now increase Rdut to 3k ohms for an expected output voltage of 3V. Rerun the Transient Simulation. Did the POS Clamp swing into action and limit the output voltage to 2V?

SPICE NETLIST