AD627A AD [Analog Devices], AD627A Datasheet - Page 13

• Manufacturer Part Number: AD627A
• Description: Micropower, Single and Dual Supply Rail-to-Rail Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

REV. A
Error Source
ABSOLUTE ACCURACY at T
DRIFT TO +85 C
had a mismatch of almost exactly 0.1% (R1 = 9999.5 , R2 =
999.76 , R3 = 1000.2 , R4 = 9997.7 ). As expected the
CMRR at dc was measured at about 84 dB (calculated value
is 85 dB). However, as the frequency increases, the CMRR
quickly degrades. For example, a 200 mV peak-peak harmonic
of the mains frequency at 180 Hz would result in an output
voltage of about 800 V. To put this in context, a 12-bit data
acquisition system with an input range of 0 V to 2.5 V, has an
LSB weighting of 610 V.
By contrast, the AD627 uses precision laser trimming of internal
resistors along with patented CMR trimming to yield a higher
dc CMRR and a wider bandwidth over which the CMRR is flat
(see Figure 20).
Total RTI Offset Voltage, mV
Input Offset Current, nA
Internal Offset Current (Homebrew Only)
CMRR, dB
Gain
Gain Drift, ppm/ C
Total RTI Offset Voltage, mV/ C
Input Offset Current, pA/ C
VIN+
VIN–
Figure 39. 0.1% Resistor Mismatch Example
9999.5
R1
350
350
AD296
999.76
1/2
+5V
–5V
R2
A1
350
350
A
= +25 C
1000.2
100mV
R3
AD627A GAIN = 9.98
+10ppm/ C
AD627 Circuit Calculation
(250 V + (1000 V/10))/100 mV
1 nA
Not Applicable
77 dB 141 ppm 2.5 V/100 mV
0.35% + 0.1%
(–75 + 10) ppm/ C
(3.0 V/ C + (10 V/ C/10))
(16 pA/ C 350
60 C/100 mV
40.2k
AD296
9997.7
1/2
+5V
1%
R
R4
G
350 /100 mV
A2
Table IV. Make vs. Buy Error Budget
AD627A
+5V
(
Figure 38. Make vs. Buy
5+(200k /R
60 C)/100 mV (33 pA/ C 350
V
60 C
OUT
G
)
)
V
2.5V
–13–
OUT
Ground Returns for Input Bias Currents
Input bias currents are those dc currents that must flow in
order to bias the input transistors of an amplifier. These are
usually transistor base currents. When amplifying “floating”
input sources such as transformers, or ac-coupled sources,
there must be a direct dc path into each input in order that the
bias current can flow. Figure 41 shows how a bias current
path can be provided for the case of transformer coupling,
capacitive ac-coupling and for a thermocouple application.
Figure 40. CMRR Over Frequency of Discrete In Amp in
Figure 39
50 ppm/ C
(2
Total Drift Error
Grand Total Error
“Homebrew”
Circuit Calculation
(180 V
20 nA
0.7 nA
(1% Match 2.5 V)/10/100 mV
1% Match
Total Absolute Error
2.5V
3.5 V/ C
120
110
100
90
80
70
60
50
40
30
20
3.15k *
350 /100 mV
350 /100 mV
1
*1% REGISTER MATCH, 50ppm/ C TRACKING
2)/100 mV
60 C
60 C)/100 mV
"HOMEBREW" IN AMP, G = 10
1/2
350 *
10
60 C)/100 mV
LT1078IS8
FREQUENCY – Hz
350 *
100
3.15k *
1/2
+5V
Total Error
AD627-ppm Homebrew–ppm
3500
3.5
3531
13500
20535
3900
2600
3.5
6504
27039
1k
LT1078IS8
10k
V
OUT
Total Error
3600
70
2.45
25000
10000
38672
3000
4200
7
7207
45879
AD627
100k