AD629
OUTPUT CURRENT AND BUFFERING
ERROR BUDGET ANALYSIS EXAMPLE 1
The AD629 is designed to drive loads of 2 kꢂ to within 2 ꢁ of
the rails but can deliver higher output currents at lower output
voltages (see Figure 1ꢀ). If higher output current is required, the
output of the AD629 should be buffered with a precision op amp,
such as the OP113, as shown in Figure 38. This op amp can swing
to within 1 ꢁ of either rail while driving a load as small as 677 ꢂ.
In the dc application that follows, the 17 A output current from
a device with a high common-mode voltage (such as a power
supply or current-mode amplifier) is sensed across a 1 ꢂ shunt
resistor (see Figure 47). The common-mode voltage is 277 ꢁ,
and the resistor terminals are connected through a long pair of
lead wires located in a high noise environment, for example,
±7 Hz/67 Hz, 447 ꢁ ac power lines. The calculations in Table ꢀ
assume an induced noise level of 1 ꢁ at 67 Hz on the leads, in
addition to a full-scale dc differential voltage of 17 ꢁ. The error
budget table quantifies the contribution of each error source.
Note that the dominant error source in this example is due to
the dc common-mode voltage.
+V
S
AD629
REF (–)
21.1kΩ
NC
1
2
3
4
8
7
6
5
0.1µF
380kΩ 380kΩ
–IN
+IN
0.1µF
0.1µF
380kΩ
V
OP113
OUT
20kΩ
REF (+)
–V
S
OUTPUT
CURRENT
AD629
REF (–)
21.1kΩ
0.1µF
–V
S
10 AMPS
200V DC
TO GROUND
NC
NC = NO CONNECT
1
2
3
4
8
7
6
5
CM
380kΩ 380kΩ
–IN
+IN
+V
Figure 38. Output Buffering Application
S
0.1µF
1Ω
SHUNT
380kΩ
A GAIN OF 19 DIFFERENTIAL AMPLIFIER
V
OUT
While low level signals can be connected directly to the –IN and
+IN inputs of the AD629, differential input signals can also be
connected, as shown in Figure 39, to give a precise gain of 19.
However, large common-mode voltages are no longer permissible.
Cold junction compensation can be implemented using a
temperature sensor, such as the AD±97.
20kΩ
REF (+)
–V
S
60Hz
0.1µF
POWER LINE
NC = NO CONNECT
Figure 40. Error Budget Analysis Example 1: VIN = 10 V Full-Scale,
VCM = 200 V DC, RSHUNT = 1 Ω, 1 V p-p, 60 Hz Power-Line Interference
+V
S
AD629
REF (–)
21.1kΩ
NC
1
2
3
4
8
7
6
5
THERMOCOUPLE
380kΩ 380kΩ
–IN
+IN
+V
0.1µF
S
380kΩ
V
OUT
V
REF
20kΩ
REF (+)
NC = NO CONNECT
Figure 39. A Gain of 19 Thermocouple Amplifier
Table 7. AD629 vs. INA117 Error Budget Analysis Example 1 (VCM = 200 V dc)
Error, ppm of FS
Error Source
AD629
INA117
AD629
INA117
ACCURACY, TA = 25°C
Initial Gain Error
(ꢀ.ꢀꢀꢀ5 × 1ꢀ)/1ꢀ V × 1ꢀ6
(ꢀ.ꢀꢀ1 V/1ꢀ V) × 1ꢀ6
(224 × 1ꢀ-6 × 2ꢀꢀ V)/1ꢀ V × 1ꢀ6
(ꢀ.ꢀꢀꢀ5 × 1ꢀ)/1ꢀ V × 1ꢀ6
(ꢀ.ꢀꢀ2 V/1ꢀ V) × 1ꢀ6
5ꢀꢀ
1ꢀꢀ
448ꢀ
5ꢀ8ꢀ
5ꢀꢀ
2ꢀꢀ
1ꢀ,ꢀꢀꢀ
1ꢀ,7ꢀꢀ
Offset Voltage
DC CMR (Over Temperature)
(5ꢀꢀ × 1ꢀ-6 × 2ꢀꢀ V)/1ꢀ V × 1ꢀ6
Total Accuracy Error
TEMPERATURE DRIFT (85°C)
Gain
1ꢀ ppm/°C × 6ꢀ°C
(2ꢀ μV/°C × 6ꢀ°C) × 1ꢀ6/1ꢀ V
1ꢀ ppm/°C × 6ꢀ°C
6ꢀꢀ
12ꢀ
72ꢀ
6ꢀꢀ
24ꢀ
84ꢀ
Offset Voltage
(4ꢀ μV/°C × 6ꢀ°C) × 1ꢀ6/1ꢀ V
Total Drift Error
RESOLUTION
Noise, Typical, ꢀ.ꢀ1 Hz to 1ꢀ Hz, μV p-p
CMR, 6ꢀ Hz
Nonlinearity
15 μV/1ꢀ V × 1ꢀ6
(141 × 1ꢀ-6 × 1 V)/1ꢀ V × 1ꢀ6
(1ꢀ-5 × 1ꢀ V)/1ꢀ V × 1ꢀ6
25 μV/1ꢀ V × 1ꢀ6
2
3
(5ꢀꢀ × 1ꢀ-6 × 1 V)/1ꢀ V × 1ꢀ6
(1ꢀ-5 × 1ꢀ V)/1ꢀ V × 1ꢀ6
Total Resolution Error
Total Error
14
5ꢀ
1ꢀ
1ꢀ
26
63
5826
11,6ꢀ3
Rev. C | Page 13 of 16
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