RG1
+IR
RO
2
6
V1
8
9
4
11
10
+
A1
+I
+0
–0
(1)
23
RIN
C
+
RO
2
V2
17
–I
3
A2
VOUT
–
RG2
C
1
12
–IR
–
VISO
106Ω
VISO
VOUT = (V1 – V2) +
IMRR
RG1 + RG2 + RIN + RO
NOTE: (1) The offset adjustment circutry and power supply connections
have been omitted for simplicity. Refer to Figure 5 for details.
FIGURE 6c. 3652 with Differential Voltage Sources.
1MΩ
IB1
I4
EOSO
λ
EOSI
RG1
RG2
I3
I1
–
–
+
+
A2
A1
–
+
I2
IB2
C (Output)
C (Input)
Optics
I1 = I2 = I3 = I4
FIGURE 7. DC Error Analysis Model for 3650.
total output error voltage due to offset voltages and bias
The effects of temperature may be analyzed by replacing the
offset terms with their corresponding temperature gradient
terms:
currents.
106
VOUT-TOTAL
=
[EOSI + (IB1 RGI – IB2 RG2)]+ EOSO (1)
RG1 + RG2
VOUT ➞∆ VOUT/∆T, EOSI ➞∆EOSI/∆T, etc.
Offset current is defined as the difference between the two
bias currents IB1 and IB2. If IB1 = IB and IB2 = IB +IOSI
For a complete analysis of the effects of temperature, gain
variations must also be considered.
106 IOS
then, for RG1 = RG2, VOUT – IB =
2
OUTPUT NOISE
The total output noise is given by:
This component of error is not a function of gain and is
therefore included as a part of EOSO specifications. The
output errors due to the output stage bias current are also
included in EOSO. This results in a very simple equation for
the total error:
EN (RMS) = √(ENIG)2 + (ENO
)
2
where EN (RMS) = Total output noise
ENI = RMS noise of the input stage
ENO = RMS noise of the output stage
G = 106/(RG1 + RG2)
106 EOSI
VOUT-TOTAL
=
+ EOSO (for RG1 = RG2).
(2)
ENO includes the noise contribution due to the optics and the
2RG1
noise currents of the output stage. Errors created by the noise
current of the input stage are insignificant compared to other
noise sources and are therefore omitted.
In summary, it should be noted that equation (2) should be
used only when RG1 = RG2. When RG1 ≠ RG2, equation (1)
applies.
®
9
3650/52
BB [ BURR-BROWN CORPORATION ]
