CS3001
CS3002
3.2.2
Gain Calculations Summary and
Recommendations
Verify the Opamp Compensation:
Verify the opamp compensation using the open-
loop gain and phase response Bode plot in
Figure 15. Plot the calculated closed loop gain
transfer function and verify the following design
criteria are met:
Condition #1: |Av| ≤ 50 and R1 ≤ 100 Ω
The Opamp is inherently stable for |Av| ≤ 50 and
R1 ≤ 100 Ω . No C2 compensation capacitor across
R2 is required.
•
Pole P1 > opamp internal 50 kHz crossover fre-
quency
•
|Av| = 1 configuration has 70° phase margin
and 20 dB gain margin.
-
-
P1 = 1 / [2π (R1||R2) • C2], where P1 = 1 MHz
To simplify the calculation, set the pole to
P1 = 1 MHz.
•
|Av| = 50 configuration has phase margin be-
tween 40° for C
≤ 100 pF and 60° for
LOAD
C
= 0 pF.
LOAD
•
•
Z1 < opamp internal 50 kHz crossover frequency
Z1 = 1 / (2π R2 • C2)
-
Condition #2: |Av| ≤ 50 and R1 > 100 Ω
Compensation capacitor C2 across R2 is required.
Calculate C2 using the following formula:
Gain margin above the open-loop gain transfer
function is required. A gain margin of +20 dB
above the open loop gain transfer function is
optimal.
•
C2 ≥ (R1 • C ) / R2, where Cin = 50 pF
in
Condition #3: |Av| > 50
Compensation capacitor C2 across R2 is required.
Calculate and verify a value for C2 using the fol-
lowing steps.
Calculate the Compensation Capacitor Value:
1) Calculate a value for C2 using the following
formula:
3.3
Powerdown (PDWN)
C2 = 1 / [2π (R1||R2) • P1], where P1 = 1 MHz
The CS3001 single amplifier provides a power-
down function on pin 1. If this pin is left open the
amplifier will operate normally. If the powerdown
is asserted low, the amplifier will go into a low
power state. There is a pull-up resistor (approxi-
mately 800 kΩ) inside the amplifier from pin 1 to
the V+ supply. The current through this pull-up re-
sistor is the main source of current drain in the
powerdown state.
To simplify the calculation, set the pole of the filter
to P1 = 1 MHz. P1 must be set higher than the
opamp’s internal 50 kHz crossover frequency.
2) Calculate a second value for C2 using the fol-
lowing formula:
C2 ≥ (R1 • C ) / R2, where Cin = 50 pF
in
3) Use the larger of the two values calculated in
steps 1 & 2.
12
DS490F9
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