INPUT CURRENTS

The LMP201X’s input currents are different than standard

bipolar or CMOS input currents in that it appears as a current

flowing in one input and out the other. Under most operating

conditions, these currents are in the picoamp level and will

have little or no effect in most circuits. These currents tend to

increase slightly when the common-mode voltage is near the

minus supply. (See the typical curves.) At high temperatures

such as 85˚C, the input currents become larger, 0.5 nA

typical, and are both positive except when the V

V

and high temperature, do not add resistance in series with

the inputs to balance the impedances. Doing this can cause

an increase in offset voltage. A small resistance such as 1

k

Ω can provide some protection against very large transients

or overloads, and will not increase the offset significantly.

PRECISION STRAIN-GAUGE AMPLIFIER

This Strain-Gauge amplifier (Figure 4) provides high gain

(1006 or ~60 dB) with very low offset and drift. Using the

resistors’ tolerances as shown, the worst case CMRR will be

greater than 108 dB. The CMRR is directly related to the

resistor mismatch. The rejection of common-mode error, at

the output, is independent of the differential gain, which is

set by R3. The CMRR is further improved, if the resistor ratio

matching is improved, by specifying tighter-tolerance resis-

tors, or by trimming.

Extending Supply Voltages and Output Swing by Using

a Composite Amplifier Configuration:

In cases where substantially higher output swing is required

with higher supply voltages, arrangements like the ones

shown in Figure 5 and Figure 6 could be used. These

configurations utilize the excellent DC performance of the

LMP201X while at the same time allow the superior voltage

and frequency capabilities of the LM6171 to set the dynamic

performance of the overall amplifier. For example, it is pos-

GBW (A

due to V

kept at about mid-point of its overall supply voltage, and its

input common mode voltage range allows the V- terminal to

be grounded in one case (Figure 5, inverting operation) and

tied to a small non-critical negative bias in another (Figure 6,

non-inverting operation). Higher closed-loop gains are also

possible with a corresponding reduction in realizable band-

width. Table 1 shows some other closed loop gain possibili-

ties along with the measured performance in each case.

20071517

FIGURE 2.

20071504

FIGURE 3.

20071518

FIGURE 4.

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