RF
RX
RK
9
2
10
D
M
6
14
RA
D
+
–
15
VOUT
A2
5
8
R1
100kΩ
13
11
A1
17
16
RB
12
7
15V
–
+
4
1
3
–
15V
RC
R2
100kΩ
O/P PWR
+
19
Pulse GEN
20
0.47µF
I/P PWR
FIGURE 5. Optional Offset Voltage Trim.
Step 2: Output stage trim (RA = RB = 20MΩ, RC = 100kΩ,
RF = 0, RX = 100kΩ, RK = ∞, R1 and R2 connected);
A1 low, A2 low gain. Adjust R2 for 0V ±1mV or
desired setting at VOUT, pin 15 (±110mV approxi-
mate total range).
pass section (0.05Hz cutoff) is formed by the 1µF capacitor
and 3MΩ resistor which are connected in series between the
output demodulator and the inverting input of the output
stage amplifier. The low-pass section (100Hz cutoff) is
formed by the 68MΩ resistor and 22pF capacitor located in
the feedback loop of the output stage. The diodes provide for
quick recovery of the high-pass filter to overvoltages at the
input. The 100kΩ pot and the 100MΩ resistor allow the
output voltage to be trimmed to compensate for increased
offset voltage caused by unbalanced impedances seen by the
inputs of the output stage amplifier.
NOTE: Other circuit component values can be used with
valid results.
APPLICATIONS
ECG AMPLIFIER
In many modern electrocardiographic systems, the patient is
not grounded. Instead, the right-leg electrode is connected to
the output of an auxiliary operational amplifier as shown in
Figure 7. In this circuit, the common-mode voltage on the
body is sensed by the two averaging resistors, R1 and R2,
inverted, amplified, and fed back to the right-leg through
resistor R4. This negative feedback drives the common-
mode voltage to a low value. The body’s displacement
current id does not flow to ground, but rather to the output
circuit of A3. This reduces the pickup as far as the ECG
amplifier is concerned and effectively grounds the patient.
Although the features of the circuit shown in Figure 6 are
important in patient monitoring applications, they may also
be useful in other applications. The input circuitry uses an
external, low quiescent current op amp (OPA177 type)
powered by the isolated power of the input stage to form a
high impedance instrumentation amplifier input (true three-
wire input). R3 and R4 give the input stage amplifier of the
3656 a noninverting gain of 10 and an inverting gain of –9.
R1 and R2 give the external amplifier a noninverting gain of
1 + 1/9. The inputs are applied to the noninverting inputs of
the two amplifiers and the composite input stage amplifier
has a gain of 10.
The value of R4 should be as large as practical to isolate the
patient from ground. The resistors R3 and R4 may be se-
lected by these equations:
The 330kΩ, 1W, carbon resistors and diodes D1 - D4 provide
protection for the input amplifiers from defibrillation pulses.
R3 = (R1/2) (VO/VCM) and R4 = (VCM – VO)/id
The output stage in Figure 6 is configured to provide a
bandpass filter with a gain of 22.7 (68MΩ/3MΩ). The high-
(–10V≤ VO ≤ +10V and –10V ≤ VCM ≤+10V)
®
11
3656
BB [ BURR-BROWN CORPORATION ]
