Datasheet

Data Sheet AD8229
Rev. B | Page 17 of 24
THEORY OF OPERATION
A3
A1 A2
Q2Q1
C1 C2
+IN
–IN
+V
S
–V
S
+V
S
–V
S
+V
S
–V
S
R3
5kΩ
R4
5kΩ
R5
5kΩ
RG–
+V
S
–V
S
OUTPUT
REF
NODE 1
NODE 2
I
B
COMPENSATION
I
B
COMPENSATION
R
G
V
B
I I
+V
S
–V
S
+V
S
–V
S
R6
5kΩ
RG+
R2
3kΩ
R1
3kΩ
09412-058
Figure 56. Simplified Schematic
ARCHITECTURE
The AD8229 is based on the classic 3-op-amp topology. This
topology has two stages: a preamplifier to provide differential
amplification followed by a difference amplifier that removes the
common-mode voltage and provides additional amplification.
Figure 56 shows a simplified schematic of the AD8229.
The first stage works as follows. To keep its two inputs matched,
Amplifier A1 must keep the collector of Q1 at a constant voltage. It
does this by forcing RG− to be a precise diode drop from –IN.
Similarly, A2 forces RG+ to be a constant diode drop from +IN.
Therefore, a replica of the differential input voltage is placed
across the gain setting resistor, R
G
. The current that flows
through this resistance must also flow through the R1 and R2
resistors, creating a gained differential signal between the A2
and A1 outputs.
The second stage is a G = 1 difference amplifier, composed of
Amplifier A3 and the R3 through R6 resistors. This stage removes
the common-mode signal from the amplified differential signal.
The transfer function of the AD8229 is
V
OUT
= G × (V
IN+
− V
IN−
) + V
REF
where:
GR
G
kΩ6
1 +=
GAIN SELECTION
Placing a resistor across the R
G
terminals sets the gain of the
AD8229, which can be calculated by referring to Table 5 or by
using the following gain equation:
1
kΩ 6
−
=
G
R
G
Table 5. Gains Achieved Using 1% Resistors
1% Standard Table Value of R
G
(Ω) Calculated Gain
6.04 k 1.993
1.5 k 5.000
665 10.02
316 19.99
121 50.59
60.4 100.34
30.1 200.34
12.1 496.9
6.04 994.4
3.01
1994.355
The AD8229 defaults to G = 1 when no gain resistor is used.
The tolerance and gain drift of the R
G
resistor should be added
to the AD8229’s specifications to determine the total gain accuracy
of the system. When the gain resistor is not used, gain error and
gain drift are minimal.
R
G
Power Dissipation
The AD8229 duplicates the differential voltage across its inputs
onto the R
G
resistor. The R
G
resistor size should be chosen to
handle the expected power dissipation.
REFERENCE TERMINAL
The output voltage of the AD8229 is developed with respect to
the potential on the reference terminal. This is useful when the
output signal must be offset to a precise midsupply level. For
example, a voltage source can be tied to the REF pin to level-
shift the output so that the AD8229 can drive a single-supply
ADC. The REF pin is protected with ESD diodes and should
not exceed either +V
S
or −V
S
by more than 0.3 V.