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參數資料
| 型號： | AD7845BR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | DAC |
| 英文描述： | LC2MOS Complete 12-Bit Multiplying DAC |
| 中文描述： | PARALLEL, WORD INPUT LOADING, 2.5 us SETTLING TIME, 12-BIT DAC, PDSO24 |
| 封裝： | SOIC-24 |
| 文件頁數： | 8/12頁 |
| 文件大小： | 193K |
| 代理商： | AD7845BR |
AD7845
REV. B
–8–
APPLI
CATION
S CIRCUITS
PROGRAMMABLE GAIN AMPLIFIER (PGA)
The AD7845 performs a PGA function when connected as in
Figure 15. In this configuration, the R-2R ladder is connected
in the amplifier feedback loop. R
FB
is the amplifier input resis-
tor. As the code decreases, the R-2R ladder resistance increases
and so the gain increases.
V
OUT
= –V
IN
×
R
DAC
D
×
1
R
FB
,
D
=
0
to
4095
4096
= –V
IN
×
R
DAC
D
×
1
R
DAC
=
–
V
IN
D
, since R
FB
= R
DAC
Figure 15. AD7845 Connected as PGA
As the programmed gain increases, the error and noise also
increase. For this reason, the maximum gain should be limited
to 256. Table III shows gain versus code.
Note that instead of using R
FB
as the input resistor, it is also
possible to use combinations of the other application resistors,
R
A
, R
B
and R
C
. For instance, if R
B
is used instead of R
FB
, the
gain range for the same codes of Table II now goes from l/2
to 128.
Table III. Gain and Error vs. Input Code for Figure 15
Digital Inputs
Gain
4096/4095
≈
1
2
4
8
16
32
64
128
256
Error (%)
1111
1000
0100
0010
0001
0000
0000
0000
0000
1111
0000
0000
0000
0000
1000
0100
0010
0001
1111
0000
0000
0000
0000
0000
0000
0000
0000
0.04
0.07
0.13
0.26
0.51
1.02
2.0
4.0
8.0
PROGRAMMABLE CURRENT SOURCES
The AD7845 is ideal for designing programmable current
sources using a minimum of external components. Figures 16
and 17 are examples. The circuit of Figure 16 drives a program-
mable current I
L
into a load referenced to a negative supply.
Figure 17 shows the circuit for sinking a programmable current,
I
L
. The same set of circuit equations apply for both diagrams.
I
L
= I
3
= I
2
+ I
1
I
1
=
D
×
|
V
IN
|
R
DAC
,
D
=
0
to
4095
4096
I
2
=
1
R
1
D
×
|
V
IN
|
R
DAC
R
FB
=
D
×
|
V
IN
|
R
1
, since R
FB
= R
DAC
I
L
=
D
×
|
V
IN
|
R
1
+
D
×
|
V
IN
|
R
DAC
=
D
×
|
V
IN
|
R
1
×
1
+
R
1
R
DAC
Note that by making R1 much smaller than R
DAC
, the circuit
becomes insensitive to both the absolute value of R
DAC
and its
temperature variations. Now, the only resistor determining load
current I
L
is the sense resistor R1.
If R1 = 100
, then the programming range is 0 mA to 100 mA,
and the resolution is 0.024 mA.
Figure 16. Programmable Current Source
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