Placing a resistor across the RG terminals" />
參數(shù)資料
型號: AD8221BR-REEL
廠商: Analog Devices Inc
文件頁數(shù): 10/24頁
文件大小: 0K
描述: IC AMP INST PREC LN 18MA 8SOIC
產(chǎn)品培訓(xùn)模塊: Power Line Monitoring
Instrument Amp Primer Overview
設(shè)計(jì)資源: Low Cost, High Voltage, Programmable Gain Instrumentation Amplifier Using AD5292 and AD8221 (CN0114)
Low Cost Programmable Gain Instrumentation Amplifier Circuit Using ADG1611 and AD620 (CN0146)
標(biāo)準(zhǔn)包裝: 2,500
放大器類型: 儀表
電路數(shù): 1
轉(zhuǎn)換速率: 2 V/µs
-3db帶寬: 825kHz
電流 - 輸入偏壓: 200pA
電壓 - 輸入偏移: 25µV
電流 - 電源: 900µA
電流 - 輸出 / 通道: 18mA
電壓 - 電源,單路/雙路(±): 4.6 V ~ 36 V,±2.3 V ~ 18 V
工作溫度: -40°C ~ 85°C
安裝類型: 表面貼裝
封裝/外殼: 8-SOIC(0.154",3.90mm 寬)
供應(yīng)商設(shè)備封裝: 8-SO
包裝: 帶卷 (TR)
AD8221
Rev. C | Page 18 of 24
GAIN SELECTION
Placing a resistor across the RG terminals set the gain of
AD8221, which can be calculated by referring to Table 6 or
by using the gain equation.
1
4
.
49
=
G
RG
Table 6. Gains Achieved Using 1% Resistors
1% Standard Table Value of R
G (Ω)
Calculated Gain
49.9 k
1.990
12.4 k
4.984
5.49 k
9.998
2.61 k
19.93
1.00 k
50.40
499
100.0
249
199.4
100
495.0
49.9
991.0
The AD8221 defaults to G = 1 when no gain resistor is used.
Gain accuracy is determined by the absolute tolerance of RG.
The TC of the external gain resistor increases the gain drift of
the instrumentation amplifier. Gain error and gain drift are kept
to a minimum when the gain resistor is not used.
LAYOUT
Careful board layout maximizes system performance. Traces
from the gain setting resistor to the RG pins should be kept as
short as possible to minimize parasitic inductance. To ensure
the most accurate output, the trace from the REF pin should
either be connected to the local ground of the AD8221, as shown
in Figure 47, or connected to a voltage that is referenced to the
local ground of the AD8221.
Common-Mode Rejection
One benefit of the high CMRR over frequency of the AD8221 is
that it has greater immunity to disturbances, such as line noise
and its associated harmonics, than do typical instrumentation
amplifiers. Typically, these amplifiers have CMRR fall-off at
200 Hz; common-mode filters are often used to compensate for
this shortcoming. The AD8221 is able to reject CMRR over a
greater frequency range, reducing the need for filtering.
A well implemented layout helps to maintain the high CMRR
over frequency of the AD8221. Input source impedance and
capacitance should be closely matched. In addition, source
resistance and capacitance should be placed as close to the
inputs as permissible.
Grounding
The output voltage of the AD8221 is developed with respect to
the potential on the reference terminal. Care should be taken to
tie REF to the appropriate local ground.
In mixed-signal environments, low level analog signals need to
be isolated from the noisy digital environment. Many ADCs
have separate analog and digital ground pins. Although it is
convenient to tie both grounds to a single ground plane, the
current traveling through the ground wires and PC board may
cause hundreds of millivolts of error. Therefore, separate analog
and digital ground returns should be used to minimize the
current flow from sensitive points to the system ground. An
example layout is shown in Figure 45 and Figure 46.
03149-
044
Figure 45. Top Layer of the AD8221-EVAL
03149-
045
Figure 46. Bottom Layer of the AD8221-EVAL
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