AD625SE 查看數據表（PDF） - Analog Devices
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AD625SE Datasheet PDF : 15 Pages
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AD625
GROUND RETURNS FOR BIAS CURRENTS
Input bias currents are those currents necessary to bias the input
transistors of a dc amplifier. There must be a direct return path
for these currents, otherwise they will charge external capaci-
tances, causing the output to drift uncontrollably or saturate.
Therefore, when amplifying “floating” input sources such as
transformers, or ac-coupled sources, there must be a dc path
from each input to ground as shown in Figure 35.
+VS
SENSE
RF
RG
AD625
VOUT
RF
LOAD
REFERENCE
TO POWER
–VS
SUPPLY
GROUND
Figure 35a. Ground Returns for Bias Currents with
Transformer Coupled Inputs
high thermoelectric potential (about 35 µV°C). This means that
care must be taken to insure that all connections (especially
those in the input circuit of the AD625) remain isothermal. This
includes the input leads (1, 16) and the gain sense lines (2, 15).
These pins were chosen for symmetry, helping to desensitize the
input circuit to thermal gradients. In addition, the user should
also avoid air currents over the circuitry since slowly fluctuating
GND VDD VSS
15 16
14
+
VIN
13
–
AD7502
+VS
AD625
–VS
10 VOUT
9
0.1F LOW
LEAKAGE
1k⍀ 11
AD711
12
+VS
SENSE
RF
RG
AD625
VOUT
RF
LOAD
REFERENCE
TO POWER
–VS
SUPPLY
GROUND
Figure 35b. Ground Returns for Bias Currents with
Thermocouple Input
+VS
RF
RG
AD625
RF
100k⍀
100k⍀
–VS
SENSE
VOUT
LOAD
REFERENCE
TO POWER
SUPPLY
GROUND
Figure 35c. Ground Returns for Bias Currents with AC
Coupled Inputs
AUTOZERO CIRCUITS
In many applications it is necessary to maintain high accuracy.
At room temperature, offset effects can be nulled by the use of
offset trimpots. Over the operating temperature range, however,
offset nulling becomes a problem. For these applications the
autozero circuit of Figure 36 provides a hardware solution.
OTHER CONSIDERATIONS
One of the more overlooked problems in designing ultralow-
drift dc amplifiers is thermocouple induced offset. In a circuit
comprised of two dissimilar conductors (i.e., copper, kovar), a
current flows when the two junctions are at different tempera-
tures. When this circuit is broken, a voltage known as the
“Seebeck” or thermocouple emf can be measured. Standard IC
lead material (kovar) and copper form a thermocouple with a
VDD
VSS
GND
AD7510DIKD
200s
ZERO PULSE
A1
A2
A3
A4
Figure 36. Auto-Zero Circuit
thermocouple voltages will appear as “flicker” noise. In SPGA
applications relay contacts and CMOS mux leads are both
potential sources of additional thermocouple errors.
The base emitter junction of an input transistor can rectify out
of band signals (i.e., RF interference). When amplifying small
signals, these rectified voltages act as small dc offset errors. The
AD625 allows direct access to the input transistors’ bases and
emitters enabling the user to apply some first order filtering to
these unwanted signals. In Figure 37, the RC time constant
should be chosen for desired attenuation of the interfering signals.
In the case of a resistive transducer, the capacitance alone work-
ing against the internal resistance of the transducer may suffice.
RF
FILTER
CAP
RG
R
+IN
–IN
C
1
+IN
+GAIN SENSE 2
RTI NULL
3
+V
4
RTI NULL
5
+GAIN DRIVE
NC 6
REF
7
–VS 8
A1
A2
10k⍀
10k⍀
10k⍀
10k⍀
A3
AD625
RF
R
FILTER
CAP
C
16 –IN
15 –GAIN SENSE
RTO
14 NULL
13
RTO
NULL
12 –GAIN DRIVE
SENSE
11
10
VOUT
VOUT
9 +VS
Figure 37. Circuit to Attenuate RF Interference
–12–
REV. D
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