廬
APPLICATION BULLETIN
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IMPROVED DEVICE NOISE PERFORMANCE FOR THE
3650 ISOLATION AMPLIFIER
By Bonnie Baker
The 3650 is an optically coupled, differential input, isola-
tion amplifier having programmable gain. Noise for the
3650 is specified to 4碌Vrms (typ) on the input stage of the
isolation barrier and 65碌Vrms (typ) on the output stage. The
gain of the 3650 is controlled using external resistors on the
input stage. In low gains, the noise performance of the 3650
is dominated by the output stage noise figure. The noise
performance in high gains is dominated by the input stage
noise. By using two OPA627s as a pre-amp to the 3650
isolation amplifier, the noise performance of the isolation
circuit is greatly enhanced.
The input bias current noise contribution and the thermal
noise of the gain resistors is relatively small and not included
in the above calculation. E
nO
includes the noise contribution
due to the optics and the noise currents of the output stage.
Because the 3650 uses optics as opposed to a carrier type
modulation technique, there is no demodulation ripple at the
output of the device.
The output-referred change in total noise vs gain is illus-
trated in Figure 2. Figure 2 graphically shows the noise
performance of the 3650 with gains from 1 to 1000. For high
En (rms) =
2 (E
RG
鈥?G)
2
+ (E
nI
鈥?G ) + E
NO2
G
鈮?/div>
(V
1
鈥?V
2
) 10
6
2 鈥?R
G
R
G
3650
11
R
IN
Input
Stage
4碌Vrms (typ)
10Hz to 10kHz
Light
Flux
Coupling
R
G
10
Output
Stage
65碌Vrms (typ)
10Hz to 10kHz
23
V
OUT
V
1
V
2
FIGURE 1. The 3650 Isolation Amplifier Has Differential Inputs and Adjustable Gain.
The 3650 has an input section, which can be gained by two
external resistors (as shown in Figure 1), and an output
section that is essentially kept in a unity gain configuration.
The 3650鈥檚 input noise performance is specified to 4碌Vrms
(typ) times the gain over a 10Hz to 10kHz range. The output
stage鈥檚 noise contribution is 65碌Vrms (typ) from 10Hz to
10kHz. The 3650 gain can be adjusted from a gain of 1 to a
gain of 1000 by adjusting the resistors, R
G
. A first order
calculation of the noise of the 3650 in various gains is shown
below.
E
n
(rms) = (2 鈥?(E
RG
鈥?G)
2
+ (E
nI
G)
2
+ (E
nO
)
2
where:
E
n
(rms) = total noise referred to output,
E
RG
= rms noise of RG,
E
nI
= rms noise of the input stage of 3650,
E
nO
= rms noise of the output stage of 3650,
10
6
G = 2鈥
G
漏
10000
Noise (RTO, 碌Vrms)
1000
100
10
1
10
Gain (V/V)
100
1000
FIGURE 2. 3650 Noise (RTO) vs Gain of the 3650
Isolation Amplifier Shown in Figure 1.
1992 Burr-Brown Corporation
AB-044A
Printed in U.S.A. March, 1993
1
2
