AD524B AD [Analog Devices], AD524B Datasheet - Page 10

• Manufacturer Part Number: AD524B
• Description: Precision Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

AD524
INPUT BIAS CURRENTS
Input bias currents are those currents necessary to bias the input
transistors of a dc amplifier. Bias currents are an additional
source of input error and must be considered in a total error
budget. The bias currents, when multiplied by the source resis-
tance, appear as an offset voltage. What is of concern in calculat-
ing bias current errors is the change in bias current with respect to
signal voltage and temperature. Input offset current is the differ-
ence between the two input bias currents. The effect of offset
current is an input offset voltage whose magnitude is the offset
current times the source impedance imbalance.
Figure 34. Indirect Ground Returns for Bias Currents
Output
Gain
2
5
10
Table I. Output Gain Resistor Values
a. Transformer Coupled
R2
5 k
1.05 k
1 k
b. Thermocouple
c. AC Coupled
AD524
AD524
AD524
–V
+V
+V
–V
+V
–V
S
S
S
S
S
S
R1, R3
2.26 k
2.05 k
4.42 k
LOAD
LOAD
LOAD
TO POWER
SUPPLY
GROUND
TO POWER
SUPPLY
GROUND
TO POWER
SUPPLY
GROUND
Nominal
Gain
2.02
5.01
10.1
–10–
Although instrumentation amplifiers have differential inputs,
there must be a return path for the bias currents. If this is not
provided, those currents will charge stray capacitances, causing
the output to drift uncontrollably or to saturate. Therefore,
when amplifying “floating” input sources such as transformers
and thermocouples, as well as ac-coupled sources, there must
still be a dc path from each input to ground.
COMMON-MODE REJECTION
Common-mode rejection is a measure of the change in output
voltage when both inputs are changed equal amounts. These
specifications are usually given for a full-range input voltage
change and a specified source imbalance. “Common-Mode
Rejection Ratio” (CMRR) is a ratio expression while “Common-
Mode Rejection” (CMR) is the logarithm of that ratio. For
example, a CMRR of 10,000 corresponds to a CMR of 80 dB.
In an instrumentation amplifier, ac common-mode rejection is
only as good as the differential phase shift. Degradation of ac
common-mode rejection is caused by unequal drops across
differing track resistances and a differential phase shift due to
varied stray capacitances or cable capacitances. In many appli-
cations shielded cables are used to minimize noise. This tech-
nique can create common mode rejection errors unless the
shield is properly driven. Figures 35 and 36 shows active data
guards that are configured to improve ac common mode rejec-
tion by “bootstrapping” the capacitances of the input cabling,
thus minimizing differential phase shift.
GROUNDING
Many data acquisition components have two or more ground
pins that are not connected together within the device. These
grounds must be tied together at one point, usually at the sys-
tem power-supply ground. Ideally, a single solid ground would
be desirable. However, since current flows through the ground
wires and etch stripes of the circuit cards, and since these paths
have resistance and inductance, hundreds of millivolts can be
generated between the system ground point and the data
100
100
100
AD711
Figure 36. Differential Shield Driver
Figure 35. Shield Driver, G
AD712
+INPUT
–INPUT
G = 100
–V
–INPUT
+INPUT
S
RG
RG
2
RG
1
2
AD524
AD524
+V
–V
+V
–V
S
S
S
S
100
REFERENCE
V
OUT
REFERENCE
V
OUT
REV. E