参数资料
型号: AD8295BCPZ-WP
厂商: Analog Devices Inc
文件页数: 11/28页
文件大小: 0K
描述: IC IN AMP PREC FE +/-18V 16LFCSP
标准包装: 64
放大器类型: 仪表
电路数: 3
转换速率: 2 V/µs
增益带宽积: 1MHz
-3db带宽: 1.2MHz
电流 - 输入偏压: 200pA
电压 - 输入偏移: 60µV
电流 - 电源: 2mA
电流 - 输出 / 通道: 18mA
电压 - 电源,单路/双路(±): 4.6 V ~ 36 V,±2.3 V ~ 18 V
工作温度: -40°C ~ 85°C
安装类型: 表面贴装
封装/外壳: 16-VQFN,CSP
供应商设备封装: 16-LFCSP(4x4)
包装: 托盘 - 晶粒
AD8295
Rev. A | Page 19 of
28
THEORY OF OPERATION
As shown in Figure 58, the AD8295 contains a precision
instrumentation amplifier, two uncommitted op amps, and a
precision resistor array. These components allow many common
applications to be wired using simple pin-strapping, directly at
the IC. This not only saves printed circuit board (PCB) space
but also improves circuit performance because both temperature
drift and resistor tolerance errors are reduced.
A1 OUT
A1 R2
A2 +IN
A2 –IN
–IN
RG
+VS
OUT
RG
+IN
–VS
REF
1
2
3
4
8
7
6
5
13
14
15
16
A1
IA
A2
R1
20k
R2
20k
A2 OUT
AD8295
A1 +IN
A1 R1
A1 –IN
12
11
10
9
4
07
34
3-
00
Figure 58. Functional Block Diagram
UNCOMMITTED OP AMPS
The AD8295 has two uncommitted op amps that can be used
independently. These op amps allow simple pin-strapping for
many common applications circuits.
Op Amp A1 has its inverting input connected to a precision 2:1
voltage divider resistor network. Because this network is internal
to the IC, these resistors are closely matched and also track each
other, with temperature variations. Op Amp A1 and the associated
resistor network can be used to create either a noninverting gain
stage of 2 or an inverting gain stage of 1 with excellent gain
accuracy and gain drift.
Op Amp A2 is a more conventional op amp, with standard
inverting and noninverting inputs and an output.
INSTRUMENTATION AMPLIFIER
Gain Selection
The transfer function of the AD8295 is
VOUT = G × (VIN+ VIN) + VREF
where placing a resistor across the RG terminals sets the gain of
the AD8295 according to the following equation:
G
R
G 1 +
=
4
.
49
Resistor values can be obtained by referring to Table 9 or by
using the following gain equation:
1
4
.
49
=
G
R
G
Table 9. Gains Achieved Using 1% Resistors
1% Standard Table Value of RG
Calculated Gain
49.9 kΩ
1.990
12.4 kΩ
4.984
5.49 kΩ
9.998
2.61 kΩ
19.93
1.00 kΩ
50.40
499 Ω
100
249 Ω
199.4
100 Ω
495
49.9 Ω
991
The AD8295 defaults to G = 1 when no gain resistor is used.
Gain accuracy is a combination of both the RG accuracy and
the accuracy listed in the specifications in Table 2, including
accuracy over temperature. Gain error and gain drift are kept
to a minimum when the gain resistor is not used.
Common-Mode Input Voltage Range
The AD8295 in-amp architecture applies gain internally and
then removes the common-mode voltage. Therefore, internal
nodes in the AD8295 experience a combination of both the
gained signal and the common-mode signal. This combined
signal can be limited by the voltage supplies even when the
individual input and output signals are not. Figure 7 through
Figure 10 show the allowable common-mode input voltage
ranges for various output voltages and supply voltages.
If Figure 7 through Figure 10 indicate that internal voltage
limiting may be an issue, the common-mode range can be
significantly improved by lowering the gain in the instrumen-
tation amplifier by one half and applying a second G = 2 stage.
Figure 59 shows how to do this amplification with the internal
circuitry of the AD8295, requiring no additional external
components.
REF
A1
+IN
–IN
R2
20k
A1 OUT
TOTAL GAIN = IN-AMP GAIN × 2
+
IN-AMP
+
RG
R1
20k
07
34
3-
0
19
Figure 59. Applying Gain in a Later Stage Allows Wider Input
Common-Mode Range
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