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I =IA =2

IA OUT

(IA – IA )

IN+ IN

GAIN

CurrentMirror

IN+

IN-

GAIN

XTR300

SBOS336C –JUNE 2005– REVISED JUNE 2011

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IA STRUCTURE, VOLTAGE MONITOR

The instrumentation amplifier has high-impedance NPN transistor inputs that do not load the output signal, which

is especially important in current output mode. The output signal is a controlled current that is multiplexed either

to the SET pin (to close the voltage output loop) or to IA

OUT

(for external access).

The principal circuit is shown in Figure 43. The two input buffer amplifiers reproduce the input difference voltage

across R

GAIN

. The resulting current through this resistor is bidirectionally mirrored to the output. That mirroring

results in the ideal transfer function of:

(6)

The accuracy and drift of R

GAIN

defines the accuracy of the voltage to current conversion. The high accuracy and

stability of the current mirrors result from a cycling chopper technique.

Figure 43. IA Block Diagram

The output current, IA

OUT

, of the instrumentation amplifier is limited to protect the internal circuitry. This current

limit has two settings controlled by the state of M2 (see Electrical Characteristics, Short-Circuit Current

specification). Note that if R

SET

is too small, the current output limitation of the instrumentation amplifier can

disrupt the closed loop of the XTR300 in voltage output mode. With M2 = low, the nominal R

GAIN

of 10kΩ allows

an input voltage of 20V

, which produces an output current of 4mA

. When using lower resistors for R

GAIN

that

can allow higher currents, the IA output current limitation must be taken into account.

CURRENT MONITOR

In current output mode (M2 = high), the XTR300 provides high output impedance. A precision current mirror

generates an exact 1/10th copy of the output current and this current is either routed to the summing junction of

the OPA to close the feedback loop (in the current output mode) or to the I

MON

pin for output current monitoring in

other operating modes.

The high accuracy and stability of this current split results from a cycling chopper technique. This design

eliminates the need for a precise shunt resistor or a precise shunt voltage measurement, which would require

high common-mode rejection performance.

During a saturation condition of the DRV output (the error flag is active), the monitor output (I

MON

) shows a

current peak because the loop opens. Glitches from the current mirror chopper appear during this time in the

monitor signal. This part of the signal cannot be used for measurement.