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參數資料
| 型號： | 3656 |
| 元件分類： | 隔離放大器 |
| 英文描述： | Transformer Coupled ISOLATION AMPLIFIER |
| 中文描述： | 變壓器耦合隔離放大器 |
| 文件頁數： | 2/17頁 |
| 文件大小： | 149K |
| 代理商： | 3656 |
3656
10
Illustrative Calculations:
The maximum input voltage is 100mV. It is desired to
amplify the input signal for maximum accuracy. Noninverting
output is desired.
Input Stage:
Step 1
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.
So, to allow for 25% input overrange without saturation at
the output, select:
G1 = 40V/V
G1 = 1 + (RF + RA) = 40
∴ R
F + RA = 39
(13)
Step 2
RA + RF forms a voltage divider with the 100k 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 2M
. For most applications, the 2M
should be sufficiently large for RA + RF. Resistances greater
than 2M
may help decrease the loading effect, but would
increase the offset voltage drift.
The voltage divider with RA + RF = 2M is 2M/(2M +
100k
) = 2/(2 + 0.1) = 95.2%, i.e., the percent loading is
4.8%.
Choose R
A + RF = 2M
(14)
Step 3
Solving equations (13) and (14)
RA = 50k and RF = 1.95M
Step 4
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.
∴R
C = RA || (RF + 100k)
= 50k
|| (1.95M + 100k)
≈ 49k
Output Stage:
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.
∴V
OUT = (1.25 0.1)(G1)(G2)
i.e., 10V = 0.125 40 G2
∴G
2 = 10V/5V = 2V/V
Step 6
G2 = 1 + (RX/RK) = 2.0
∴R
X/RK = 1.0
∴R
X = RK
(15)
Step 7
The resistance seen by the + input terminal of the output
stage amplifier A
2 (pin 13) is the output resistance 100k of
the output demodulator. The resistance seen by the (–) input
terminal of A
2 (pin 14) should be matched to the resistance
seen by the + input terminal.
The resistance seen by pin 14 is the parallel combination of
RX and RK.
∴R
X || RK = 100k
(R
X RK/(RX + RK) = 100k
RK/[1 +(RK/RX)] = 100k
(16)
Step 8
Solving equations (15) and (16) RK = 20k and RX =
200k
.
Step 9
The output demodulator must be loaded equal to the input
demodulator.
∴R
B = RA + RF = 2M
(See equation (14) above in Step 2).
Use the resistor values obtained in Steps 3, 4, 8 and 9, and
connect the 3656 as shown in Figure 3.
OFFSET TRIMMING
Figure 5 shows an optional offset voltage trim circuit. It is
important that RA + RF = RB.
CASE 1: Input and output stages in low gain, use output
potentiometer (R
2) only. Input potentiometer (R1)
may be disconnected. For example, unity gain
could be obtained by setting R
A = RB = 20M, RC
= 100k
, R
F = 0, RX = 100k, and RK = ∞.
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
RF = 2M, RB = 2M returned to pin 17, RA =
20k
, R
X = 100k, and RK = ∞.
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 A
1 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 1: Input stage trim (RA = RC = 20k, RI = RB = 20M.
RX = 100k, RK = ∞, R2 disconnected); A1 high, A2
low gain. Adjust R1 for 0V ±5mV or desired setting
at VOUT, pin 15.
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