REV. A
OP471
–10–
Noise Measurement - Current Noise Density
The test circuit shown in Figure 10 can be used to measure current
noise density. The formula relating the voltage output to current
noise density is:
i
e
G
40nV / Hz
R
n
nOUT
S
=
Ê
Ë
Á
ˆ
¯
˜
-
()
2
2
where:
G = gain of 10,000
R
S
= 100 kW source resistance
Capacative Load Driving and Power Supply Considerations
The OP471 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 OP471.
R2
100k
R3
1.24k
OP471
DUT
R5
8.06k
OP27E
R4
200
e
n
OUT TO
SPECTRUM ANALYZER
R1
5
GAIN = 10,000
V
S
= 15V
Figure 10. Current Noise Density Test Circuit
R1
100*
*SEE TEXT
R3
50
OP471
C5
0.1F
*
C4
10F
+
V–
V
OUT
C
L
1000pF
C1
200pF
R2
V
IN
PLACE SUPPLY DECOUPLING
CAPACITORS AT OP471
C3
0.1F
C2
10F
+
V+
Figure 11. Driving Large Capacitive Loads
In the standard feedback amplifier, the op amp’s output resistance
combines with the load capacitance to form a lowpass 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 load capacitances of up
to 1,000 pF when used with the OP471.
In applications where the OP471’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 applications 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 disconnected. 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 applica-
tions, only one diode clamp is required per board or system.
8V/s
OP471
R
f
Figure 12. Pulsed Operation
Unity-Gain Buffer Applications
When R
f
£ 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.
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 R
f
≥ 500 W, the output
is capable of handling the current requirements (I
L
£ 20 mA at
10 V); the amplifier will stay in its active mode and a smooth
transition will occur.
When R
f
> 3 kW, a pole created by R
f
and the amplifier’s input
capacitance (2.6 pF) creates additional phase shift and reduces
phase margin. A small capacitor (20 pF to 50 pF) in parallel with
R
f
helps eliminate this problem.
APPLICATIONS
Low Noise Amplifier
A simple method of reducing amplifier noise by paralleling
amplifiers is shown in Figure 13. Amplifier noise, depicted in
Figure 14, is around 5 nV/÷Hz @ 1 kHz (R.T.I.). Gain for each
paralleled amplifier and the entire circuit is 100. The 200 W
resistors limit circulating currents and provide an effective output
resistance of 50 W. The amplifier is stable with a 10 nF capacitive
load and can supply up to 30 mA of output drive.
