AD625 AD [Analog Devices], AD625 Datasheet - Page 12

• Manufacturer Part Number: AD625
• Description: Programmable Gain Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

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.
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
100k
R
R
R
R
R
R
R
R
R
100k
G
G
G
F
F
F
F
F
F
AD625
AD625
AD625
+V
–V
+V
–V
+V
–V
S
S
S
S
S
S
REFERENCE
REFERENCE
REFERENCE
SENSE
SENSE
SENSE
LOAD
LOAD
LOAD
V
TO POWER
V
TO POWER
V
TO POWER
GROUND
GROUND
GROUND
OUT
SUPPLY
OUT
SUPPLY
OUT
SUPPLY
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
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.
V
ZERO PULSE
+
–
IN
15
GND
V
V
DD
SS
16
14
13
200 s
R
F
GND V
+GAIN SENSE
AD7502
+GAIN DRIVE
A1
FILTER
DD
CAP
+V
RTI NULL
RTI NULL
C
V
SS
REF
–V
+IN
NC
S
1
2
3
4
5
6
7
8
R
A2
AD625
10k
+IN
10k
A1
AD625
R
A3
A3
G
+V
–V
10k
S
A2
S
10k
–IN
A4
R
0.1 F LOW
16
15
14
13
12
11
10
LEAKAGE
9
SENSE
AD711
–GAIN SENSE
V
+V
–IN
–GAIN DRIVE
RTO
NULL
RTO
NULL
OUT
S
FILTER
CAP
AD7510DIKD
C
V
OUT
1k
12
11
R
F
9
10 V
OUT