Datasheet

C
F
=
¨
¨
©
§
R
F
+ 2R
IN
RF
2
¨
¨
©
§
C
L
R
OUT
R
S
= R
OUT
R
IN
R
F
LM6211
www.ti.com SNOSAH2C –FEBRUARY 2006–REVISED MARCH 2013
The values for R
S
and C
F
are decided by ensuring that the zero attributed to C
F
lies at the same frequency as the
pole attributed to C
L
. This ensures that the effect of the second pole on the transfer function is compensated for
by the presence of the zero, and that the ROC is maintained at 20 dB/decade. For the circuit shown in Figure 36
the values of R
S
and C
F
are given by Equation 1. Table 1 shows different values of R
S
and C
F
that need to be
used for maintaining stability with different values of C
L
, as well as the phase margins to be expected. R
F
and R
IN
are assumed to be 10 kΩ, R
L
is taken as 2 kΩ, while R
OUT
is taken to be 60Ω.
(1)
Table 1.
C
L
(pF) R
S
(Ω) C
F
(pF) Phase Margin (°)
250 60 4.5 39.8
300 60 5.4 49.5
500 60 9 53.1
Although this methodology provides circuit stability for any load capacitance, it does so at the price of bandwidth.
The closed loop bandwidth of the circuit is now limited by R
S
and C
F
.
Compensation by External Resistor
In some applications it is essential to drive a capacitive load without sacrificing bandwidth. In such a case, in the
loop compensation is not viable. A simpler scheme for compensation is shown in Figure 37. A resistor, R
ISO
, is
placed in series between the load capacitance and the output. T110his introduces a zero in the circuit transfer
function, which counteracts the effect of the pole formed by the load capacitance, and ensures stability.
Figure 37. Compensation By Isolation Resistor
The value of R
ISO
to be used should be decided depending on the size of C
L
and the level of performance
desired. Values ranging from 5Ω to 50Ω are usually sufficient to ensure stability. A larger value of R
ISO
will result
in a system with lesser ringing and overshoot, but will also limit the output swing and the short circuit current of
the circuit.
Stability and Input Capacitance
In certain applications, for example I-V conversion, transimpedance photodiode amplification and buffering the
output of current-output DAC, capacitive loading at the input of the op amp can endanger stability. The
capacitance of the source driving the op amp, the op amp input capacitance and the parasitic/wiring capacitance
contribute to the loading of the input. This capacitance, C
IN
, interacts with the feedback network to introduce a
peaking in the closed loop gain of the circuit, and hence causes instability.
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