Dual Per-Pin Parametric Measurement Units

18

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Applications Information

In force-voltage (FV) mode, the output FORCE_ voltage

is directly proportional to the input control voltage. In

force-current (FI) mode, the current flowing out of the

FORCE_ output is proportional to the input control volt-

age. Positive current flows out of the PMU.

In force-nothing (FN) mode, the FORCE_ output is high

impedance.

In measure-current (MI) mode, the voltage at the MSR_

output is directly proportional to the current exiting the

FORCE_ output. Positive current flows out of the PMU.

In measure-voltage (MV) mode, the voltage at the

MSR_ output is directly proportional to the voltage at

the SENSE_ input.

Current-Sense-Amplifier Offset

Voltage Input

IOS is a buffered input to the current-sense amplifier.

The current-sense amplifier converts the input control

voltage (IN0_ or IN1_) to the forced DUT current (FI)

AND converts the sensed DUT current to the MSR_ out-

put voltage (MI). When IOS equals zero relative to DUT-

GND (the GND voltage at the DUT, which the

level-setting DACs and the ADC are presumed to use

as a ground reference), the nominal voltage range that

corresponds to ± full-scale current is -4V to +4V. Any

voltage applied to the IOS input adds directly to this

control input/measure output voltage range, i.e., apply-

ing +4V to IOS forces the voltage range that corre-

sponds to ± full-scale current from 0 to +8V.

The following equations determine the minimum and

maximum currents for each current range correspond-

ing to the input voltage or measure voltage:

Choose IOS so the limits of the MSR_ output do not go

supplies of +10V and -5V, limit the MSR_ output to -2.2V

and +7.2V. Therefore, set IOS between +1.8V and +3.2V.

The MSR_ output could clip if IOS is not within this range.

Use these general equations for the limits on IOS:

Current Booster for Highest Current Range

An external buffer amplifier can be used to provide a

current range greater than the MAX9949/MAX9950

maximum output current (Figure 5). This function oper-

ates as follows.

A digital output decoded from the range select bits,

EXTSEL_, indicates when to activate the booster. The

R_COM output serves as an input to an external buffer

through a 50

Ω current-limit series resistor. Each side of

the external current-sense resistor feeds back to RXA_

and RXD_. Ensure that the buffer circuit enters a high-Z

output state when not selected. Any leakage in the

buffer adds to the leakage of the PMU.

Voltage Clamps

The voltage clamps limit the FORCE_ output and oper-

ate over the entire specified current range. Set the

clamp voltages externally at CLHI_ and CLLO_. The

voltage at the FORCE_ output triggers the clamps inde-

pendent of the voltage at the SENSE_ input. When

enabled, the clamps function in both FI and FV modes.

Current Limit

The current-limiting circuitry on the FORCE_ output

ensures a well-behaved MSR_ output for currents

between the full current range and the current limits, i.e.,

for currents greater than the full-scale current, the MSR_

voltage is greater than +4V and for currents less than the

full-scale current, the MSR_ voltage is less than -4V.

Independent Control of the Feedback

Switch and the Measure Switch

Two single-pole-double-throw (SPDT) switches deter-

mine the mode of operation of the PMU. One switch

FORCE_

R_COM

50

Ω

RXA_

RXD_

INTERNAL TO MAX9949/MAX9950

EXTSEL_

Figure 5. External Current Boost