tions, the voltage reference output trim procedure can be
used to null offset errors at the output of the RCV420. The
voltage reference trim circuit is discussed under “Voltage
Reference.”
When the voltage reference is not used for level shifting or
when large offset adjustments are required, the circuit in
Figure 3 can be used for offset adjustment. A low impedance
on the Rcv Com pin is required to maintain high CMR.
ZERO ADJUSTMENT
Level shifting the RCV420 output voltage can be achieved
using either the Ref In pin or the Rcv Com pin. The
disadvantage of using the Ref In pin is that there is an 8:1
voltage attenuation from this pin to the output of the RCV420.
Thus, use the Rcv Com pin for large offsets, because the
voltage on this pin is seen directly at the output. Figure 4
shows the circuit used to level-shift the output of the RCV420
using the Rcv Com pin. It is important to use a low-output
impedance amplifier to maintain high CMR. With this method
of zero adjustment, the Ref In pin must be connected to the
Rcv Com pin.
MAINTAINING COMMON-MODE REJECTION
Two factors are important in maintaining high CMR: (1)
resistor matching and tracking (the internal resistor network
does this) and (2) source impedance. CMR depends on the
accurate matching of several resistor ratios. The high accu-
racies needed to maintain the specified CMR and CMR
temperature coefficient are difficult and expensive to reli-
ably achieve with discrete components. Any resistance im-
balance introduced by external circuitry directly affects
CMR. These imbalances can occur by: mismatching sense
resistors when gain is decreased, adding resistance in the
feedback path when gain is increased, and adding series
resistance on the Rcv Com pin.
The two sense resistors are laser-trimmed to typically match
within 0.01%; therefore, when adding parallel resistance to
decrease gain, take care to match the parallel resistance on
each sense resistor. To maintain high CMR when increasing
the gain of the RCV420, keep the series resistance added to
the feedback network as small as possible. Whether the Rcv
Com pin is grounded or connected to a voltage reference for
level shifting, keep the series resistance on this pin as low as
possible. For example, a resistance of 20Ω on this pin
degrades CMR from 86dB to approximately 80dB. For
applications requiring better than 86dB CMR, the circuit
shown in Figure 5 can be used to adjust CMR.
PROTECTING THE SENSE RESISTOR
The 75Ω sense resistors are designed for a maximum con-
tinuous current of 40mA, but can withstand as much as
250mA for up to 0.1s (see absolute maximum ratings).
There are several ways to protect the sense resistor from
–In
C
T
+In
1
15
2
3
13
+15V
12
OPA237
±150mV adjustment at output.
1kΩ
–15V
100kΩ
100kΩ
RCV420
5
14
V
O
FIGURE 3. Optional Output Offset Nulling Using External
Amplifier.
–In
C
T
+In
Use 10V Ref for +
and 10V Ref with INA105 for –.
1
15
2
3
5
13
12
V
ZERO
OPA237
10kΩ
50kΩ
10kΩ
1kΩ
V
O
= (0.3125)(I
IN
) + V
ZERO
14
V
O
2
+10V
3
RCV420
13
1k
Ω
1kΩ
RCV420
10
Procedure:
1. Connect CMV to C
T
.
2. Adjust potentiometer for near zero
at the output.
11
5
INA105
1
6
–10V
OPA237
1kΩ
200Ω
CMR
Adjust
±5V adjustment
at output.
FIGURE 4. Optional Zero Adjust Circuit.
FIGURE 5. Optional Circuit for Externally Trimming CMR.
®
RCV420
6
BURR-BROWN [ BURR-BROWN CORPORATION ]
