Illustrative Calculations:
Step 6
G2 = 1 + (RX/RK) = 2.0
The maximum input voltage is 100mV. It is desired to
amplify the input signal for maximum accuracy. Noninverting
output is desired.
RX/RK = 1.0
RX = RK
(15)
Input Stage:
Step 7
Step 1
The resistance seen by the + input terminal of the output
stage amplifier A2 (pin 13) is the output resistance 100kΩ of
the output demodulator. The resistance seen by the (–) input
terminal of A2 (pin 14) should be matched to the resistance
seen by the + input terminal.
G1 max = 5V/max Input Signal = 5V 0.1V = 50V/V
With the above gain of 50V/V, if the input ever exceeds
100mV, it would drive the output to saturation. Therefore, it
is good practice to allow reasonable input overrange.
The resistance seen by pin 14 is the parallel combination of
RX and RK.
So, to allow for 25% input overrange without saturation at
the output, select:
RX || RK = 100kΩ
(RX • RK/(RX + RK) = 100kΩ
G1 = 40V/V
G1 = 1 + (RF + RA) = 40
RK/[1 +(RK/RX)] = 100kΩ
(16)
RF + RA = 39
Step 2
(13)
Step 8
Solving equations (15) and (16) RK = 20kΩ and RX
200kΩ.
=
RA + RF forms a voltage divider with the 100kΩ output
resistance of the demodulator. To limit the voltage divider
loading effect to no more than 5%, RA + RF should be
chosen to be at least 2MΩ. For most applications, the 2MΩ
should be sufficiently large for RA + RF. Resistances greater
than 2MΩ may help decrease the loading effect, but would
increase the offset voltage drift.
Step 9
The output demodulator must be loaded equal to the input
demodulator.
RB = RA + RF = 2MΩ
(See equation (14) above in Step 2).
The voltage divider with RA + RF = 2MΩ is 2MΩ/(2MΩ +
100kΩ) = 2/(2 + 0.1) = 95.2%, i.e., the percent loading is
4.8%.
Use the resistor values obtained in Steps 3, 4, 8 and 9, and
connect the 3656 as shown in Figure 3.
Choose RA + RF = 2MΩ
Step 3
(14)
OFFSET TRIMMING
Figure 5 shows an optional offset voltage trim circuit. It is
important that RA + RF = RB.
Solving equations (13) and (14)
RA = 50kΩ and RF = 1.95MΩ
Step 4
CASE 1: Input and output stages in low gain, use output
potentiometer (R2) only. Input potentiometer (R1)
may be disconnected. For example, unity gain
could be obtained by setting RA = RB = 20MΩ, RC
= 100kΩ, RF = 0, RX = 100kΩ, and RK = ∞.
The resistances seen by the + and – input terminals of the
input amplifier A1 should be closely matched in order to
minimize offset voltage due to bias currents.
CASE 2: Input stage in high gain and output stage in low
gain, use input potentiometer (R1) only. Output
potentiometer (R2) may be disconnected. For
example, GT = 100 could be obtained by setting
RC = RA || (RF + 100kΩ)
= 50kΩ || (1.95MΩ + 100kΩ)
≈ 49kΩ
RF = 2MΩ, RB = 2MΩ returned to pin 17, RA
20kΩ, RX = 100kΩ, and RK = ∞.
=
Output Stage:
CASE 3: When it is necessary to perform a two-stage
precision trim (to maintain a very small offset
change under conditions of changing temperature
and changing gain in A1 and A2), use step 1 to
adjust the input stage and step 2 for the output
stage. Carbon composition resistors are accept-
able, but potentiometers should be stable.
Step 5
VOUT = VIN MAX • G1 • G2
As discussed in Step 1, it is good practice to provide 25%
input overrange.
So we will calculate G2 for 10V output and 125% of the
maximum input voltage.
Step 1: Input stage trim (RA = RC = 20kΩ, RI = RB = 20MΩ.
RX = 100kΩ, RK = ∞, R2 disconnected); A1 high, A2
low gain. Adjust R1 for 0V ±5mV or desired setting
at VOUT, pin 15.
VOUT = (1.25 • 0.1)(G1)(G2)
i.e., 10V = 0.125 • 40 • G2
G2 = 10V/5V = 2V/V
®
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3656
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