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參數(shù)資料
| 型號(hào)： | HS7516 |
| 英文描述： | 16-Bit Multiplying DACs |
| 中文描述： | 16位乘法數(shù)模轉(zhuǎn)換器 |
| 文件頁(yè)數(shù)： | 6/8頁(yè) |
| 文件大小： | 130K |
| 代理商： | HS7516 |
Corporation
SIGNAL PROCESSING EXCELLENCE
132
Figure 4. Microprocessor Interface to SP7514
D0
D1
D2
D3
D4
D5
D6
D7
CLK
74273
VREF
(+ 25V MAX)
LSB
15
14
13
12
11
10
9
HS3160
SP7514/
7516
8
7
6
5
4
3
2
D0
D1
D2
D3
D4
D5
D6
D7
74273
CLK
MSBGND
LATCHES
ADDRESS DECODER
G2A
74LS138
G2B
C
B
A
D0
D1
D2
D3
D4
D5
D6
D7
+
–
200
470
3
DD
V
400
WR
BDSEL
A2
A1
A0
VREF
VDD
+
R
I01
I02
UNIPOLAR MODE
(2-QUADRANT)
6
2
3
A1
0 VOUT
(1-2
- N
)
R0S
F
HS3160,
small values for C
must be used. Resis-
tor R
can be added, this will parallel R
decreasing
the effective resistance. If C
is reduced the band-
width will be increased and settling time decreased.
However a system penalty for lowering C
is to
increase noise gain. The tradeoff is noise vs. set-
tling time. If R
is added then a large value (1
μ
F or
greater) non-polarized capacitor C
should be added
in series with R
to eliminate any DC drifts. If
settling time is not important, eliminate R
p
and C
p
,
and adjust C
f
to prevent overshoot.
Output Offset
In most applications, the output of the DAC is fed
into an amplifier to convert the DAC’s current
output to voltage. A little known and not com-
monly discussed parameter is the linearity error
versus offset voltage of the output amplifier. All
CMOS DAC’s must operate into a virtual ground,
i.e., the summing junction of an op amp. Any
amplifier’s offset from the amplifier will appear as
an error at the output (which can be related to
LSB’s of error).
Most all CMOS DAC’s currently available are
implemented using an R-2R ladder network. The
formula for nonlinearity is typically 0.67mV/mV
(not derived here). However the
SP7516
has a
coefficient of only 0.065mV/mV
. This is due to
the decoding technique described earlier. CMOS
DAC applications notes (including this one) al-
ways show a potentiometer used to null out the
amplifier’s offset. If an amplifier is chosen having
‘pretrimmed’ offset it may be possible to eliminate
this component. Consider the following calcula-
tions:
1.
Using LF441A amplifier (low power - 741 pinout)
2.
Specified offset: 0.5mV max
3.
Temperature coefficient of input offset: 10
μ
V/
°
C max
V
OS
max (0
°
C to 70
°
C)
= 0.5mV + (70
μ
V)10
= 1.2mV
Add'l nonlinearity (max)
= 1.2mV x 0.065mV/mV
= 78
μ
V (1/2 LSB @ 16 Bits!)
Where: 78
μ
V = 1/2 LSB @ 16 Bits (10V range)
Via the above configuration, the
SP7516/HS3160
can be used to divide an analog signal by digital
code (i.e. for digitally controlled gain). The trans-
fer function is given in
Table 2,
where the value of
each bit is 0 or 1. Division by all “0”s is undefined
and causes the op amp to saturate.
Following the decoded section of the DAC a
standard binary weighted R-2R approach is used.
This divides each of the 16 levels (or 6.25% of
F.S.) into 4096 discrete levels (the 12 LSB’s).
Output Capacitance
The
SP7516/HS3160
have very low output ca-
pacitance (C
). This is specified both with all
switches ON and all switches OFF. Output capaci-
tance varies from 50pF to 100pF over all input
codes. This low capacitance is due in part to the
decoding technique used. Smaller switches are
used with resulting less capacitance. Three impor-
tant system characteristics are affected by C
and
C
; namely digital feedthrough, settling time,
and bandwidth. The DAC output equivalent cir-
cuit can be represented as shown in
Figure 1
.
Digital feedthrough is the change in analog output
due to the toggling conditions on the converter
input data lines when the analog input V
is at
0V. The
SP7516/HS3160
very low C
and there-
fore will yield low digital feedthrough. Inputs to
the DAC can be buffered. This input latch with
microprocessor control is shown in
Figure 4
.
Settling time is directly affected by C
. In
Figure
1
, C
combines with R
to add a pole to the open
loop response, reducing bandwidth and causing
excessive phase shift - which could result in
ringing and/or oscillation. A feedback capaci-
tor, C
must be added to restore stability. Even with
C
, there is still a zero-pole mismatch due to R
C
O
which is code dependent. This code dependent
mismatch is minimized when C
R
= R
C
. How-
ever C
must now be made larger to compensate for
worst case
R
C
- resulting in reduced bandwidth
and increased settling time. With the
SP7516/
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