Advance Information
MT88E45
1.00
0.95
0.90
0.85
0.80
Gain Ratio
0.75
0.70
0.668
0.65
0.60
0.55
0.531
0.50
3.0
3.5
4.0
Nominal Vdd (Volts)
4.5
5.0
Gain ratio for
Bellcore GS1, GS2
ETSI GS2 op amps
0.794
Gain ratio for ETSI
GS1 op amp
Figure 8 - Gain Ratio as a Function of Nominal Vdd
Gain Setting Resistor Calculation Example for Figure 8:
•
•
For the desired nominal Vdd, use Figure 8 to determine approximate A
v
.
For the GS1 op-amp, start with the 0dB gain setting resistor values of R5
0dB
, R6
0dB
and R7
0dB
. In
Figure 7 these values are 53K7, 60K4 and 464K respectively. Keep C1,C2,R1,R2,R3,R4 as in Figure 7
to maintain the highpass corner frequency constant for all gain settings.
For the desired gain setting of A
v
:
R7
Av
= R7
0dB
x A
V
Scaled for desired gain. Choose
the closest standard resistor value as R7
Av
.
Actual A
v
from now on is R7
Av
/R7
0db
Scaled for good common mode range. Choose
the closest standard resistor value as R5
Av
.
•
R5
Av
= R5
0dB
x A
V
•
Calculate R6
Av
so that R5
Av
=R6
Av
|| R7
Av
. Choose
the closest standard resistor value as R6
Av
.
Repeat for R
10
, R
11
, R
12
for the GS2 op-amp.
1/R6
Av
= 1/R5
Av
- 1/R7
Av
Example:
• For a gain of -3.5dB, A
v
=10
-3.5/20
= 0.668
• R7
-3.5dB
= 464K x 0.668 = 309K9, the closest standard resistor value is 309K.
A
v
is now 309K/464K = 0.666
• R5
-3.5dB
= 53K6 x 0.666 = 35K7, the closest standard resistor value is 35K7.
Therefore R6
-3.5dB
is calculated to be 40K4, the closest standard resistor value is 40K2.
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