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CS3011-IS

Precision Low-voltage Amplifier; DC to 1 kHz

Cirrus Logic 

CS3011

CS3012

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 CLOAD ≤ 100 pF and 60° for

CLOAD = 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 • Cin) / 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 14. Plot the calculated closed loop gain

transfer function and verify the following design cri-

teria 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 for-

mula:

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 • Cin) / R2, where Cin = 50 pF

3) Use the larger of the two values calculated in

steps 1 & 2.

3.3 Powerdown (PDWN)

The CS3011 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 enters a powered down

state. There is a pull-up resistor (approximately

800 k ohm) inside the amplifier from pin 1 to the V+

supply. The current through this pull-up resistor is

the main source of current drain in the powerdown

state.

DS597F5

13

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