Advance Information
MT88E45
1.00
0.95
0.90
0.85
0.80
0.75
0.70
Gain ratio for
Bellcore GS1, GS2
ETSI GS2 op amps
0.794
Gain ratio for ETSI
GS1 op amp
0.668
0.65
0.60
0.55
0.50
0.531
3.0
3.5
4.0
Nominal Vdd (Volts)
4.5
5.0
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
, R6
and R7
. In
0dB
0dB
0dB
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 = R7
x A
V
Scaled for desired gain. Choose
the closest standard resistor value as R7 .
Av
0dB
0dB
Av
Actual A from now on is R7 /R7
v
Av
0db
R5 = R5
x A
Scaled for good common mode range. Choose
the closest standard resistor value as R5 .
Av
V
Av
1/R6 = 1/R5 - 1/R7
Calculate R6 so that R5 =R6 || R7 . Choose
Av Av Av Av
Av
Av
Av
the closest standard resistor value as R6 .
Av
•
Repeat for R10, R11, R12 for the GS2 op-amp.
Example:
-3.5/20
•
•
For a gain of -3.5dB, A =10
= 0.668
v
R7
= 464K x 0.668 = 309K9, the closest standard resistor value is 309K.
-3.5dB
A is now 309K/464K = 0.666
v
•
R5
= 53K6 x 0.666 = 35K7, the closest standard resistor value is 35K7.
-3.5dB
Therefore R6
is calculated to be 40K4, the closest standard resistor value is 40K2.
-3.5dB
13
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