Cirrus Logic CS3002 - Manual (Page 9)

CS3002
DS490F10 9
3.2 Open-loop Gain and Stability
3.2.1 Discussion
The CS3002 achieves ultra-high open-loop gain.
Figure 14 illustrates the amplifier in a non-inverting gain
configuration. The open-loop gain and phase plots
indicate that the amplifier is stable for closed-loop gains
less than 50V/V and R1
 100 
. For a gain of 50, the
phase margin is between 40  and 60 depending upon
the loading conditions. As shown in Figure 15, on
page 9 , the operational amplifier has an input
capacitance at the + and – signal inputs of typically
50pF. This capacitance adds an additional pole in the
loop gain transfer function at a frequency defined using
Equation 1:
where
R = R1  R2; the parallel combination of R1 and R2
A higher value for R produces a pole at a lower
frequency, thus reducing the phase margin. Resistor R1
is recommended to be less than or equal to 100

,
which results in a pole at 30MHz or higher. If a higher
value of R1 is desired, compensation capacitor C2
should be added in parallel with resistor R2. Capacitor
C2 should be chosen using Equation 2:
The feedback capacitor C2 is required for closed-loop
gains greater than 50V/V. The capacitor introduces a
pole P
1
and a zero Z
1
in the loop gain transfer function
T(s), see Equation 3
f
1
2 R C
in

-------------------------- = [Eq. 1]
R2 C2  R1 C
in
  [Eq. 2]
R1
R2
Vin
Vo
R
S
CS3002
Figure 14. Non-inverting Gain Configuration
50 pF
50 pF
R1
R2
Vin
Vo
C2
C
in
C in
Choose C2 so that R2  C2  R1  C
in
CS3002
Figure 15. Non-inverting Gain Configuration with Compensation
T s  
1
s
Z
1
------ +
 
 
–
1
s
P
1
------ +
 
 
------------------------- A
ol
 = [Eq. 3]