OP470
CAPACITIVE LOAD DRIVING AND POWER
SUPPLY CONSIDERATIONS
R1
The OP470 is unity-gain stable and is capable of driving large
capacitive loads without oscillating. Nonetheless, good supply
bypassing is highly recommended. Proper supply bypassing
reduces problems caused by supply line noise and improves the
capacitive load driving capability of the OP470.
In the standard feedback amplifier, the op amp’s output resistance
combines with the load capacitance to form a low pass filter that
adds phase shift in the feedback network and reduces stability.
A simple circuit to eliminate this effect is shown in Figure 11.
The added components, C1 and R3, decouple the amplifier
from the load capacitance and provide additional stability. The
values of C1 and R3 shown in Figure 11 are for a load capaci-
tance of up to 1000 pF when used with the OP470.
V+
C2
10 F
+
C3
0.1 F
OP470
2V/ s
Figure 12. Pulsed Operation
During the fast feedthrough-like portion of the output, the input
protection diodes effectively short the output to the input, and a
current, limited only by the output short-circuit protection, will
be drawn by the signal generator. With Rf
£
500
W,
the output
is capable of handling the current requirements (IL < 20 mA at
10 V); the amplifier will stay in its active mode and a smooth
transition will occur.
When Rf > 3 kW, a pole created by Rf and the amplifier’s input
capacitance (2 pF) creates additional phase shift and reduces
phase margin. A small capacitor (20 pF to 50 pF) in parallel
with Rf helps eliminate this problem.
R2
R1
C1
1000pF
APPLICATIONS
Low Noise Amplifier
V
OUT
C
L
1000pF
V
IN
OP470
100
*
R3
50
C4
10 F
+
*
C5
0.1 F
*SEE
TEXT
A simple method of reducing amplifier noise by paralleling
amplifiers is shown in Figure 13. Amplifier noise, depicted in
Figure 14, is around 2 nV/÷Hz @ 1 kHz (R.T.I.). Gain for each
paralleled amplifier and the entire circuit is 1000. The 200
W
resistors limit circulating currents and provide an effective out-
put resistance of 50
W.
The amplifier is stable with a 10 nF
capacitive load and can supply up to 30 mA of output drive.
+15V
V
IN
R1
50
R3
200
V–
PLACE SUPPLY DECOUPLING
CAPACITORS AT OP470
Figure 11. Driving Large Capacitive Loads
In applications where the OP470’s inverting or noninverting
inputs are driven by a low source impedance (under 100
W)
or
connected to ground, if V+ is applied before V–, or when V is
disconnected, excessive parasitic currents will flow. Most applica-
tions use dual tracking supplies and with the device supply pins
properly bypassed, power-up will not present a problem. A source
resistance of at least 100
W
in series with all inputs (Figure 11)
will limit the parasitic currents to a safe level if V– is discon-
nected. It should be noted that any source resistance, even 100
W,
adds noise to the circuit. Where noise is required to be kept at a
minimum, a germanium or Schottky diode can be used to clamp
the V- pin and eliminate the parasitic current flow instead of
using series limiting resistors. For most applications, only one
diode clamp is required per board or system.
When Rf
£
100
W
and the input is driven with a fast, large
signal pulse(> 1 V), the output waveform will look as shown
in Figure 12.
UNITY-GAIN BUFFER APPLICATIONS
1/4
OP470E
R2
50k
–15V
R4
50
1/4
OP470E
R5
50k
R6
200
V
OUT
= 1000V
IN
R9
200
R7
50
1/4
OP470E
R8
50k
R10
50
1/4
OP470E
R11
50k
R12
200
Figure 13. Low Noise Amplifier
–12–
REV. B
AD [ ANALOG DEVICES ]
