MAX9949/MAX9950
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:
VMAXCURRENT = VIOS + 4V
VMINCURRENT = VIOS - 4V
Choose IOS so the limits of the MSR_ output do not go
closer than 2.8V to either VEE or VCC. For example, with
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:
Minimum VIOS = VEE + 6.8V
Maximum VIOS = VCC - 6.8V
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
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
determines whether the sensed DUT current or DUT
voltage feeds back to the input (sensing), and thus
FORCE_
REXTBOOST
R_COM
50
RXA_
RXD_
INTERNAL TO MAX9949/MAX9950
AV = +2
EXTSEL_
Figure 5. External Current Boost