They say that if you are using your amplifier to drive a capacitive =
load=20
(Figure 1, CLOAD), a good rule of thumb is to isolate =
the=20
amplifier from the capacitor with a 50 or 100 Ohm resistor =
(RISO).=20
This additional resistor may stop your operational amplifier (op =
amp)=20
from oscillating.
Figure 1. An amplifier with a capacitive load may require a =
resistor=20
between the amplifier output and the load capacitor.
The use of 50 or 100 Ohms (RISO) may not work every time. =
A key=20
question to ask is, =E2=80=9CWhat do I do if my CLOAD exceeds =
the recommended=20
data sheet op amp capacitive load value?=E2=80=9D
If you can=E2=80=99t find any data sheet guidance or your load =
capacitance=20
(CLOAD) does exceed data sheet recommendations, the answer to =
this=20
question depends on the:
- amplifier gain bandwidth product (GBWP or fU)
- amplifier=E2=80=99s open-loop output resistance =
(RO)
- capacitor load value (CLOAD)
The frequency versus gain graph in Figure 1 shows what happens =
to the=20
amplifier=E2=80=99s open loop gain curve when an RISO and =
CLOAD=20
are added at the output of your amplifier. If you use these three =
variables, you=20
will find the appropriate RISO value.
Here are rules when determining the value of RISO:
&n=
bsp; &nb=
sp; =20
(Eq. 1)
&n=
bsp; =20
(Eq. 2)
These rules ensure that the circuit is stable.
An appropriate application to apply this concept is where you are =
driving the=20
input to a SAR-ADC. In this case, you want the signal to settle within =
the=20
acquisition time (tACQ) of the converter. In Equation 3, K is =
an ADC=20
time-constant multiplier that provides half-LSB accuracy. For a 16-bit=20
converter, such as the ADS7=
886,=20
K equals 11.78.
&n=
bsp; =20
(Eq. 3)
Let=E2=80=99s put these formulas to use. Working with the following =
parameters:
- fU =3D 50 MHz
- R0 =3D 30 ohms
- Gain =3D 1 V/V
- tACQ =3D 300 nsec
- CIN =3D 21 pF
According to the OPA=
365=20
data sheet, a load of 100 pF will product an overshoot of 30% (Figure =
2).
Figure 2. OPA=
365 Overshoot=20
versus capacitive load
To solve this overshoot problem, equations 1, 2, and 3 can help.
- With equation 1, RISO =3D> 3.33 =
Ohms
- With equation 2, RISO =3D> 30.97 Ohms
- With equation 3, RISO ~ 61.96 =
Ohms
Given these three equations, RISO must be equal to 61.9 =
Ohms (0.1%=20
tolerance).
Some manufacturers include a stability versus capacitive load =
typical=20
performance curve in their data sheet. They may even provide a line =
in the=20
electrical performance table. All this information is very helpful, of =
course.=20
However, you can avoid amplifier oscillations or excessive overshoot =
with the=20
calculations used in this post.
References
- For more information, download these datasheet: OPA=
365,=20
ADS7=
886
- Baker, Bonnie, =E2=80=9CStart=20
with the right op amp when driving SAR ADCs,=E2=80=9D EDN, October =
16, 2008
- Baker, Bonnie, =E2=80=9CCharge=20
your SAR-converter inputs,=E2=80=9D EDN, May 11, 2006