AD623
The output voltage at Pin 6 is measured with respect to the
potential at Pin 5. The impedance of the reference pin is 100 kΩ,
so in applications requiring V/I conversion, a small resistor
between Pins 5 and 6 is all that is needed.
POS SUPPLY
7
+
INVERTING
–
2
4
50k⍀
50k⍀
50k⍀
50k⍀
1
–
+
OUT
6
GAIN
50k⍀
50k⍀
REF
5
8
7
4
Figure 38. Gain Nonlinearity (G = –100, 15 ppm/Div)
–
NON-
INVERTING
3
+
V+
NEG SUPPLY
Figure 40. Simplified Schematic
(V+) –0.5
The bandwidth of the AD623 is reduced as the gain is increased,
since all the amplifiers are of voltage feedback type. At unity
gain, it is the output amplifier that limits the bandwidth. There-
fore even at higher gains the AD623 bandwidth does not roll off
as quickly.
(V+) –1.5
(V+) –1.5
APPLICATIONS
Basic Connection
(V–) +0.5
V–
Figure 41 shows the basic connection circuit for the AD623.
The +VS and –VS terminals are connected to the power supply.
The supply can be either bipolar (VS = ±2.5 V to ±6 V) or
single supply (–VS = 0 V, +VS = 3.0 V to 12 V). Power supplies
should be capacitively decoupled close to the devices power
pins. For best results, use surface mount 0.1 µF ceramic chip
capacitors and 10 µF electrolytic tantalum capacitors.
0
0.5
1.5
1
2
OUTPUT CURRENT – mA
Figure 39. Output Voltage Swing vs. Output Current
THEORY OF OPERATION
The input voltage, which can be either single-ended (tie either
–IN or +IN to ground) or differential is amplified by the pro-
grammed gain. The output signal appears as the voltage difference
between the Output pin and the externally applied voltage on
the REF input. For a ground referenced output, REF should be
grounded.
The AD623 is an instrumentation amplifier based on a modified
classic three op amp approach, to assure single or dual supply
operation even at common-mode voltages at the negative supply
rail. Low voltage offsets, input and output, as well as absolute
gain accuracy, and one external resistor to set the gain, make the
AD623 one of the most versatile instrumentation amplifiers in
its class.
GAIN SELECTION
The input signal is applied to PNP transistors acting as voltage
buffers and providing a common-mode signal to the input
amplifiers (Figure 40). An absolute value 50 kΩ resistor in each
of the amplifiers’ feedback assures gain programmability.
The AD623’s gain is resistor programmed by RG, or more pre-
cisely, by whatever impedance appears between Pins 1 and 8.
The AD623 is designed to offer accurate gains using 0.1%–1%
tolerance resistors. Table I shows required values of RG for
various gains. Note that for G = 1, the RG terminals are uncon-
The differential output is
nected (RG =
by using the formula
ϱ). For any arbitrary gain, RG can be calculated
100 kΩ
VO = 1+
VC
RG
RG = 100 kΩ/(G – 1)
The differential voltage is then converted to a single-ended
voltage using the output amplifier, which also rejects any common-
mode signal at the output of the input amplifiers.
REFERENCE TERMINAL
The reference terminal potential defines the zero output voltage
and is especially useful when the load does not share a precise
ground with the rest of the system. It provides a direct means of
injecting a precise offset to the output. The reference terminal is
also useful when bipolar signals are being amplified as it can be
used to provide a virtual ground voltage. The voltage on the
reference terminal can be varied from –VS to +VS.
Since all the amplifiers can swing to either supply rails, as well
as have their common-mode range extended to below the nega-
tive supply rail, the range over which the AD623 can operate is
further enhanced (Figures 19 and 20).
REV. C
–11–
ADI [ ADI ]
