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參數(shù)資料
| 型號(hào)： | AD625 |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | 運(yùn)動(dòng)控制電子 |
| 英文描述： | Programmable Gain Instrumentation Amplifier(儀器用放大器) |
| 中文描述： | 可編程增益儀表放大器（儀器用放大器） |
| 文件頁數(shù)： | 13/14頁 |
| 文件大?。?/td> | 549K |
| 代理商： | AD625 |
AD625
REV. B
–13–
Figure 40. Time to 0.01% of a 20 V Step Input for
SPGA with AD625
DETERMINING SPGA RESISTOR NETWORK VALUES
The individual resistors in the gain network can be calculated
sequentially using the formula given below. The equation deter-
mines the resistors as labeled in Figure 41. The feedback resis-
tors and the gain setting resistors are interactive, therefore; the
formula must be a series where the present term is dependent on
the preceding term(s). The formula
R
F
i
+
1
=
(20
k
–
R
F
j
j
=
0
1
∑
)(1–
G
i
G
i
=
1
)
G
0
=
1
R
F
0
=
0
can be used to calculate the necessary feedback resistors for any
set of gains. This formula yields a network with a total resistance
of 40 k
. A dummy variable (j) serves as a counter to keep a
running total of the preceding feedback resistors. To illustrate
how the formula can be applied, an example similar to the
calculation used for the resistor network in Figure 38 is exam-
1) Unity gain is treated as a separate case. It is implemented
with separate 20 k
feedback resistors as shown in Figure
41. It is then ignored in further calculations.
2) Before making any calculations it is advised to draw a resistor
network similar to the network in Figure 41. The network
will have (2
×
M) + 1 resistors, where M = number of gains.
For Figure 38 M = 3 (4, 16, 64), therefore, the resistor string
will have 7 resistors (plus the two 20 k
“side” resistors for
unity gain).
3) Begin all calculations with
G
0
= 1 and
R
F
0
= 0.
R
F
1
= (20
k
–
R
F
0
) (1–1/4):
R
F
0
= 0
∴
R
F
1
= 15
k
R
F
2
= [20
k
–(
R
F
0
+
R
F
1
)] (1–4/16):
R
F
0
+
R
F
1
= 15
k
∴
R
F
2
= 3.75
k
R
F
3
= [20
k
–(
R
F
0
+
R
F
1
+
R
F
2
)] (1–16/64):
R
F
0
+
R
F
1
+
R
F
2
= 18.75
k
∴
R
F
3
= 937.5
4) The center resistor (R
G
of the highest gain setting), is deter-
mined last. Its value is the remaining resistance of the 40 k
string, and can be calculated with the equation:
R
G
=
(40
k
– 2
R
F
j
j
=
0
∑
)
R
G
=
40
k
–
2
(R
F0
+ R
F
1
+ R
F
2
+
R
F
3
)
40
k
–3
9.375
k
= 625
5) If different resistor values are desired, all the resistors in the
network can be scaled by some convenient factor. However,
raising the impedance will increase the RTO errors, lowering
the total network resistance below 20 k
can result in ampli-
fier instability. More information on this phenomenon is
given in the RPGA section of the data sheet. The scale factor
will not affect the unity gain feedback resistors. The resistor
network in Figure 38 has a scaling factor of 650/625 = 1.04,
if this factor is used on R
F1
,
R
F
2
,
R
F
3
, and R
G
, then the resis-
tor values will match exactly.
6) Round off errors can be cumulative, therefore, it is advised to
carry as many significant digits as possible until all the values
have been calculated.
Figure 41. Resistors for a Gain Setting Network
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