AD8230
Rev. B | Page 11 of 16
THEORY OF OPERATION
Auto-zeroing is a dynamic offset and drift cancellation
technique that reduces input-referred voltage offset to the
μV level and voltage offset drift to the nV/°C level. A further
advantage of dynamic offset cancellation is the reduction of
low frequency noise, in particular the 1/f component.
The AD8230 is an instrumentation amplifier that uses an
auto-zeroing topology and combines it with high common-
mode signal rejection. The internal signal path consists of an
active differential sample-and-hold stage (preamp) followed by
a differential amplifier (gain amp). Both amplifiers implement
auto-zeroing to minimize offset and drift. A fully differential
topology increases the immunity of the signals to parasitic noise
and temperature effects. Amplifier gain is set by two external
resistors for convenient TC matching.
The signal sampling rate is controlled by an on-chip, 6 kHz
oscillator and logic to derive the required nonoverlapping
clock phases. For simplification of the functional description,
two sequential clock phases, A and B, are shown to distinguish
the order of internal operation, as depicted in
Figure 30 and
Figure 31, respectively.
–
–
+
+
V
REF
C
HOLD
V
OU
V
+IN
V
–IN
C
HOLD
C
SAMPLE
R
F
R
G
PREAMP
GAIN AMP
DIFF
CM
–V
S
–V
S
05063-030
Figure 30. Phase A of the Sampling Phase
During Phase A, the sampling capacitors are connected to the
inputs. The input signal’s difference voltage, V
DIFF
, is stored
across the sampling capacitors, C
SAMPLE
. Because the sampling
capacitors only retain the difference voltage, the common-mode
voltage is rejected. During this period, the gain amplifier is not
connected to the preamplifier so its output remains at the level
set by the previously sampled input signal held on C
HOLD
, as
shown in
Figure 30.
–
–
+
+
V
REF
C
HOLD
V
OU
V
+IN
V
–IN
C
HOLD
C
SAMPLE
R
F
R
G
PREAMP
GAIN AMP
DIFF
CM
–V
S
–V
S
05063-031
Figure 31. Phase B of the Sampling Phase
In Phase B, the differential signal is transferred to the hold
capacitors refreshing the value stored on C
HOLD
. The output of
the preamplifier is held at a common-mode voltage determined
by the reference potential, V
REF
. In this manner, the AD8230 is
able to condition the difference signal and set the output voltage
level. The gain amplifier conditions the updated signal stored
on the hold capacitors, C
HOLD
.
SETTING THE GAIN
Two external resistors set the gain of the AD8230. The gain is
expressed in the following equation:
)2(1
G
F
R
R
Gain +=
(1)
2
6
1
7
5
8
4
3
AD8230
V
OUT
–V
S
+
S
0.1µF
R
G
R
F
R
G
V
REF
1
V
REF
2
0.1µF
10µF
10µF
05063-032
Figure 32. Gain Setting
Table 5. Gains Using Standard 1% Resistors
Gain R
F
R
G
Actual Gain
2 0 Ω (short) None 2
10 8.06 kΩ 2 kΩ 10
50 12.1 kΩ 499 Ω 50.5
100 9.76 kΩ 200 Ω 99.6
200 10 kΩ 100 Ω 202
500 49.9 kΩ 200 Ω 501
1000 100 kΩ 200 Ω 1002
Figure 32 and Table 5 provide an example of some gain settings.
As
Table 5 shows, the AD8230 accepts a wide range of resistor
values. Because the instrumentation amplifier has finite driving
capability, ensure that the output load in parallel with the sum
of the gain setting resistors is greater than 2 kΩ.
R
L
||(R
F
+ R
G
) > 2 kΩ (2)
Offset voltage drift at high temperature can be minimized by
keeping the value of the feedback resistor, R
F
, small. This is due
to the junction leakage current on the R
G
pin, Pin 7. The effect
of the gain setting resistor on offset voltage drift is shown in
Figure 33. In addition, experience has shown that wire-wound
resistors in the gain feedback loop may degrade the offset
voltage performance.
