CS3001_08 Datasheet, PDF (12/16 Pages) Cirrus Logic – Precision Low-voltage Amplifier; DC to 2 kHz

CS3001
CS3002
3.2.2 Gain Calculations Summary and
Recommendations
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.
• |Av| = 1 configuration has 70° phase margin
and 20 dB gain margin.
• |Av| = 50 configuration has phase margin be-
tween 40° for C_LOAD ≤ 100 pF and 60° for
C_LOAD = 0 pF.
Condition #2: |Av| ≤ 50 and R1 > 100 Ω
Compensation capacitor C2 across R2 is required.
Calculate C2 using the following formula:
• C2 ≥ (R1 • C_in) / R2, where Cin = 50 pF
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:
• Pole P1 > opamp internal 50 kHz crossover fre-
quency
- P1 = 1 / [2π (R1||R2) • C2], where P1 = 1 MHz
- To simplify the calculation, set the pole to
P1 = 1 MHz.
• Z1 < opamp internal 50 kHz crossover frequency
- Z1 = 1 / (2π R2 • C2)
• 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.
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:
C2 = 1 / [2π (R1| |R2) • P1], where P1 = 1 MHz
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_in) / R2, where Cin = 50 pF
3) Use the larger of the two values calculated in
steps 1 & 2.
3.3 Powerdown (PDWN)
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.
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DS490F8