LT6350CDDPBF datasheet(13/28 Pages) LINER

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LT6350

6350f

–

RINT

OUT1

OUT2

VIN

+–

–IN1

+IN1

+IN2

OP AMP 1

OP AMP 2

6350 F03

Figure 3. General Conﬁguration

Notice that the output common mode voltage is determined

simply by the voltage at +IN2. However, since the voltage

appliedat+IN2doesnotaffectthevoltageattheVOUT1output,

a differential offset voltage will develop for VA = 0 when V1

does not equal V2. The value of the offset voltage will be

2 • (V1 – V2), as can be seen in Equation 2. For lowest

differential offset, therefore, the input signal to pin +IN1,

VIN, should be centered around the common mode voltage

applied to pin +IN2. Often this voltage is provided by the

ADC reference output. When the input is so centered and

V1 = V2, Equation 2 reduces to:

VOUTDIFF = 2 • VA • (1+RF/RG)

The simple connection described in the Basic Connections

section can be seen as a special case of the general circuit

in Figure 3 where RF is a short circuit, RG is an open circuit,

and the voltage at VIN is centered around the voltage V2. If

differential gain greater than two is needed, the values of RF

and RG can be adjusted in accordance with Equation (2).

Additional information about feedback networks is given

in the next section and in the Input Ampliﬁer (Op Amp 1)

Feedback Components section.

Inverting Gain Connections/Interfacing to High

Voltage Signals

Although the previous examples have assumed the input

signal is applied at +IN1, it is also possible to use the input

op amp in an inverting conﬁguration by ﬁxing the voltage

VIN and applying the input signal at V1 of Figure 3. Using the

input op amp in the inverting conﬁguration ﬁxes its input

common mode voltage at the voltage VIN, which allows the

input signal at V1 to traverse a swing beyond the LT6350

supply rails. To avoid unwanted differential offsets in this

conﬁguration VIN should be chosen such that:

VIN = V2/(1+(RF/RG))

Then Equation (1) reduces to:

VOUTDIFF = –2 • V1 • (RF/RG))

Choosing RF = RG with the input at V1 leads to the gain

of –2 conﬁguration.

A practical application for the inverting gain conﬁguration is

interfacing a high voltage op amp to a 5V differential SAR

ADC.AsseeninFigure4,anindustrialapplicationmighthave

OPERATION

–

RINT

OUT1

OUTMAX

OUTMIN

OUT2

VIN

OUTMAX

OUTMIN

OUTHV

OUTHVMAX

OUTHVNOM

OUTHVMIN

–IN1

+IN1

+IN2

–15V

+15V

OP AMP 1

OP AMP 2

6350 F04

SIGNAL

HIGH VOLTAGE OP AMP

Figure 4. Interfacing to High Voltage Signals

sensed signals coming through an op amp running from

±15V rails. The LT6350 can easily interface the high voltage

opamptoa5VADCbyusingtheinvertinggainconﬁguration.

For a clean interface, three conditions must be met:

1. VOUTDIFF = 0 when OUTHV is centered at OUTHVNOM.

2. VOUT1 = VOUTCM = V2 when OUTHV is centered at

OUTHVNOM.

3. Full-scale signals at OUTHV are translated at the

output of the LT6350 into the appropriate full-scale

range for the ADC.

Applying the above constraints to the design Equations

(1) to (3) gives values for the ratio of RF to RG and for

the value of VIN:

R R

OUT

MAX

MIN

HVMAX

HVMIN

)/(

)

−−

HVNOM

OUT

R R

/ (

(

)) (

)/(

(

))

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