Datasheet

Data Sheet AD5522
Rev. E | Page 41 of 64
FORCE CURRENT (FI) MODE
In force current (FI) mode, the voltage at the FIN DAC is
converted to a current and is applied to the DUT. The feedback
path is the measure current amplifier, feeding back the voltage
measured across the sense resistor. MEASOUTx reflects the
voltage measured across the DUT (see Figure 55).
For the suggested current ranges, the maximum voltage drop
across the sense resistors is ±1 V. However, to allow for error
correction, there is some overrange available in the current
ranges. The maximum full-scale voltage range that can be
loaded to the FIN DAC is ±11.5 V. The forced current can be
calculated as follows:
FI = 4.5 × VREF × ((DAC_CODE − 32,768)/2
16
)/(R
SENSE
×
MI_Amplifier_Gain)
where:
FI is the forced current.
R
SENSE
is the selected sense resistor.
MI_Amplifier_Gain is the gain of the measure current
instrumentation amplifier. This gain can be set to 5 or
10 via the serial interface.
The I
SENSE
amplifier is biased by the offset DAC output voltage in
such a way as to center the measure current output regardless of
the voltage span used.
In the ±200 μA range with the 5 kΩ sense resistor and an I
SENSE
gain of 10, the maximum current range possible is ±225 μA.
Similarly, for the other current ranges, there is an overrange of
12.5% to allow for error correction.
DUTGND
DUT
R
SENSE
FIN
INTERNAL RANGE SELECT
(±5µA, ±20µA, ±200µA, ±2mA)
DAC
–
–
–
–
–
–
+
–
+
+
+
+
+
+
×5 OR
×10
×1
AGND
VMID TO
CENTER
I RANGE
06197-034
R
SENSE
(UP TO
±80mA)
MEASVHx
EXTMEASIHx
EXTMEASILx
FOHx
EXTFOHx
CFFx
MEASOUTx
FORCE
AMPLIFIER
MEASURE CURRENT
AMPLIFIER
×1 OR
×0.2
4k Ω
2kΩ
4k Ω
Figure 55. Forcing Current, Measuring Voltage