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LT1996

1996f

High Voltage CM Difference Amplifiers

This class of difference amplifier remains to be discussed.

Figure 14 shows the basic circuit on the top. The effective

input voltage range of the circuit is extended by the fact

that resistors RT attenuate the common mode voltage

seen by the op amp inputs. For the LT1996, the most

useful resistors for RG are the M9 and P9 50kΩ resistors,

because they do not have diode clamps to the supplies and

therefore can be taken outside the supplies. As before, the

input CM of the op amp is the limiting factor and is set by

the voltage at the op amp +input, VINT. By superposition

we can write:

VINT = VEXT • (RF||RT)/(RG + RF||RT) + VREF • (RG||RT)/

(RF + RG||RT) + VTERM • (RF||RG)/(RT + RF||RG)

Solving for VEXT:

VEXT = (1 + RG/(RF||RT)) • (VINT – VREF • (RG||RT)/

(RF + RG||RT) – VTERM • (RF||RG)/(RT + RF||RG))

Given the values of the resistors in the LT1996, this

equation has been simplified and evaluated, and the re-

sulting equations provided in Table 6. As before, substi-

tuting VCC – 1.2 and VEE + 1 for VLIM will give the valid

upper and lower common mode extremes respectively.

Following are sample calculations for the case shown in

Figure 14, right-hand side. Note that P81 and M81 are

terminated so row 3 of Table 6 provides the equation:

MAX VEXT = 91/9 • (VCC – 1.2V) – VREF/9 – 9 • VTERM

= (10.11) • (10.8) – 0.11(2.5) – 9(10) =

18.9V

and:

MIN VEXT = 91/9 • (VEE + 1V) – VREF/9 – 9 • VTERM

= (10.11)(1) – 0.11(2.5) – 9(10) = –80.2V

but this exceeds the 60V absolute maximum rating of the

P9, M9 pins, so –60V becomes the de facto negative

common mode limit. Several more examples of high CM

circuits are shown in Figures 15, 16, 17 for various

supplies.

Figure 14. Extending CM Input Range

4pF

–

VIN

(= VEXT)

VIN

–

VIN

–

VOUT

VOUT = GAIN • (VIN

+ – VIN–)

GAIN = RF/RG

–

1996 F14

450k

LT1996

REF

HIGH CM VOLTAGE DIFFERENCE AMPLIFIER

INPUT CM TO OP AMP IS ATTENUATED BY

RESISTORS RT CONNECTED TO VTERM.

HIGH NEGATIVE CM VOLTAGE DIFFERENCE AMPLIFIER

IMPLEMENTED WITH LT1996.

RF = 450k, RG = 50k, RT 5.55k, GAIN = 9

VTERM = 10V = VCC = 12V, VREF = 2.5V, VEE = 0V.

VCC

VEE

VTERM

VREF

INPUT CM RANGE

= –60V TO 18.9V

10V

12V

2.5V

450k/81

450k/27

450k/9

450k/81

450k/27

450k/9

4pF

Table 6. HighV CM Connections Giving Difference Gains

for the LT1996

Max, Min VEXT

Noise

(Substitute VCC – 1.2,

Gain

VIN+

VIN–

Gain

VEE + 1 for VLIM)

10/9 • VLIM - VREF/9

P27, M27

37/9 • VLIM – VREF/9 – 3 • VTERM

P81, M81

91/9 • VLIM – VREF/9 – 9 • VTERM

P27||P81

118 118/9 • VLIM – VREF/9 – 12 • VTERM

M27||M81

APPLICATIO S I FOR ATIO

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