LT6350
13
6350fc
+–
–
+
+–
+
–
+–
RS
RG
RF
RINT
OUT1
OUT2
VIN
V1
V2
VA
+
–
–IN1
+IN1
+IN2
OP AMP 1
OP AMP 2
6350 F03
8
1
2
4
5
Figure 3. General Configuration
Noticethattheoutputcommonmodevoltageisdetermined
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,shouldbecenteredaroundthecommonmodevoltage
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
differentialgaingreaterthantwoisneeded,thevaluesofRF
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
VINandapplyingtheinputsignalatV1ofFigure3.Usingthe
input op amp in the inverting configuration fixes its input
commonmodevoltageatthevoltageVIN,whichallowsthe
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
–
+
+–
+
–
+–
RF
RG
RINT
OUT1
OUTMAX
OUTMIN
OUT2
VIN
V2
OUTMAX
OUTMIN
OUTHV
V2
OUTHVMAX
OUTHVNOM
OUTHVMIN
–IN1
+IN1
+IN2
–15V
+15V
OP AMP 1
OP AMP 2
6350 F04
SIGNAL
HIGH VOLTAGE OP AMP
8
1
2
5
4
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:
RR
OUT
V
FG
MAXMIN
HVMAXHVMIN
I
/(
)/()
=
NNF
GHVNOM
GF
VR R
OUT
RR
=+
++
21
1
/( (/
)) ()/( (/ ))