Datasheet

LT6350
13
6350fc
For more information www.linear.com/LT6350
+
–
–
+
+
–
+
–
+
–
R
S
R
G
R
F
R
INT
R
INT
OUT1
OUT2
V
IN
V1 V2
V
A
+ –
–IN1
+IN1
+IN2
OP AMP 1
OP AMP 2
6350 F03
8
1
2
4
5
Figure 3. General Configuration
Notice that the output common mode voltage is determined
simply by the voltage at +IN2. However, since the voltage
applied at +IN2 does not affect the voltage at the V
OUT1
output,
a differential offset voltage will develop for V
A
= 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,
V
IN
, 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:
V
OUTDIFF
= 2 • V
A
• (1+R
F
/R
G
)
The simple connection described in the Basic Connections
section can be seen as a special case of the general circuit
in Figure 3 where R
F
is a short circuit, R
G
is an open circuit,
and the voltage at V
IN
is centered around the voltage V2. If
differential gain greater than two is needed, the values of R
F
and R
G
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
V
IN
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 V
IN
, which allows the
input signal at V1 to traverse a swing beyond the LT6350
supply rails. To avoid unwanted differential offsets in this
configuration V
IN
should be chosen such that:
V
IN
= V2/(1+(R
F
/R
G
))
Then Equation (1) reduces to:
V
OUTDIFF
= –2 • V1 • (R
F
/R
G
))
Choosing R
F
= R
G
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. As seen in Figure 4, an industrial application might have
operaTion
–
+
+
–
+
–
+
–
R
F
R
G
R
INT
R
INT
OUT1
OUT
MAX
OUT
MIN
OUT2
V
IN
V2
V2
OUT
MAX
OUT
MIN
OUT
HV
V2
OUT
HVMAX
OUT
HVNOM
OUT
HVMIN
–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
op amp to a 5V ADC by using the inverting gain configuration.
For a clean interface, three conditions must be met:
1. V
OUTDIFF
= 0 when OUT
HV
is centered at OUT
HVNOM
.
2. V
OUT1
= V
OUTCM
= V2 when OUT
HV
is centered at
OUT
HVNOM
.
3. Full-scale signals at OUT
HV
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 R
F
to R
G
and for
the value of V
IN
:
RR OUT OUT OUT OUT
V
FG MAXMIN HVMA
XH
VMIN
I
/( )/()=− −
NNFGHVNOM GF
VRR OUT RR=+
++
21 1/( (/)) ()/( (/))