+

-

+

-

VOUT

5V

R1

R2

R2

R1

10k, 0.1%

2k, 1%

2k, 1%

10k, 0.1%

R3

20:

LMP2014MT

SNOSAK6B – DECEMBER 2004 – REVISED MARCH 2013

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INPUT CURRENTS

The LMP2014'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 70°C,

the input currents become larger, 0.5 nA typical, and are both positive except when the VCM is near V

−. If

operation is expected at low common-mode voltages 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 32) 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 resistors, or by trimming.

Figure 32.

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 33 and Figure 34 could be used. These configurations utilize the excellent DC performance

of the LMP2014 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 possible to achieve ±12V output swing

with 300 MHz of overall GBW (AV = 100) while keeping the worst case output shift due to VOS less than 4 mV.

The LMP2014 output voltage is 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 33, inverting operation) and tied to a

small non-critical negative bias in another (Figure 34, non-inverting operation). Higher closed-loop gains are also

possible with a corresponding reduction in realizable bandwidth. Table 1 shows some other closed loop gain

possibilities along with the measured performance in each case.

12

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