ETC AB-044

APPLICATION BULLETIN
®
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IMPROVED DEVICE NOISE PERFORMANCE FOR THE
3650 ISOLATION AMPLIFIER
By Bonnie Baker
The input bias current noise contribution and the thermal
noise of the gain resistors is relatively small and not included
in the above calculation. EnO 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 3650 is an optically coupled, differential input, isolation amplifier having programmable gain. Noise for the
3650 is specified to 4µVrms (typ) on the input stage of the
isolation barrier and 65µVrms (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 output-referred change in total noise vs gain is illustrated in Figure 2. Figure 2 graphically shows the noise
performance of the 3650 with gains from 1 to 1000. For high
2 (ERG • G)2 + (EnI • G ) + ENO2
En (rms) =
G≅
RG
RG
V1
(V1 – V2 ) 106
2 • RG
3650
11
10
V2
RIN
Light
Flux
Coupling
Input
Stage
Output
Stage
23
VOUT
65µVrms (typ)
10Hz to 10kHz
4µVrms (typ)
10Hz to 10kHz
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’s input noise performance is specified to 4µVrms
(typ) times the gain over a 10Hz to 10kHz range. The output
stage’s noise contribution is 65µVrms (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, RG. A first order
calculation of the noise of the 3650 in various gains is shown
below.
Noise (RTO, µVrms)
10000
En (rms) = (2 • (ERG • G)2 + (EnIG)2 + (EnO)2
1992 Burr-Brown Corporation
100
10
1
where:
En (rms) = total noise referred to output,
ERG = rms noise of RG,
EnI = rms noise of the input stage of 3650,
EnO = rms noise of the output stage of 3650,
106
G = 2•R
G
©
1000
10
100
1000
Gain (V/V)
FIGURE 2. 3650 Noise (RTO) vs Gain of the 3650
Isolation Amplifier Shown in Figure 1.
AB-044A
Printed in U.S.A. March, 1993
En (rms) = (2 • (EOPA627 • G)2 + (EnI)2 + (ENO)2
G≅
OPA627
106
2 RG1
1+
2RF
RG2
3650
V1
RG2
RF
RG1
RF
RG1
Light
Flux
Coupling
RIN
Input
Stage
OPA627
V2
Output
Stage
VOUT
65µVrms (typ)
10Hz to 10kHz
4µVrms (typ)
10Hz to 10kHz
0.5µVrms (typ)
10Hz to 10kHz
FIGURE 3. By Using Two OPA627s, Noise Performance is Improved for High Gains of the 3650.
values of RG (or low input stage gains) the total noise
referred to the output of the 3650 is dominated by the noise
in the output stage, which is specified to 65µVrms (typ). As
RG decreases in value, the gain of the 3650 increases and
eventually the noise in the input stage dominates due to the
increase in gain. As shown in Figure 2, the effects of the
input stage noise starts to dominate as the 3650 gain increases above 10V/V.
The change in total noise referred to output vs gain of the
circuit in Figure 3 is shown graphically in Figure 4. The
effects of the input stage noise starts to dominate as the 3650
gain increases above 50V/V, which is a significant improvement. If the application requires that the isolation amplifier
have a gain of 2100, the improvement in noise performance
is 3.4.
Noise is a typical problem confronting many isolation applications. By using a differential input stage constructed with
two OPA627s, the noise performance of the 3650 is greatly
improved for higher gains.
If the 3650 is applied in a low gain configuration, the noise
referred to output will be optimized; however, it is possible
to improve the noise performance in mid to high gains by
using a pre-gain stage to the 3650. Figure 3 illustrates a
configuration using the 3650 and two OPA627 amplifiers to
improve the noise performance of the overall isolation
solution. Here the OPA627 is selected because of its low
noise performance characteristics; however, a variety of
amplifiers could be used instead, depending on the noise
requirements of the particular application. Two op amps are
configured at the input to the 3650 to preserve the differential input and the programmable gain features that the 3650
offers. The total output noise calculation for this circuit is
given by:
Noise (RTO, µVrms)
10000
1000
100
En (rms) = (2 • (EOPA267 • G)2 + (EnI)2 + (EnO)2
10
where:
En (rms) = total noise referred to output,
EOPA627 = rms noise the OPA627 operational
amplifier,
EnI = rms noise of the input stage of 3650,
EnO = rms noise of the output stage of 3650,
RF
106
G=
• 1+2•
2 • RG1
RG2
1
10
100
1000
Gain (V/V)
FIGURE 4. 3650 with OPA627 Pre-Amp Noise (RTO)
vs Gain of the 3650 Isolation Amplifier with
OPA627s Used for Gain as Shown in Figure 3.
2