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參數資料
| 型號： | AD7840JN |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | DAC |
| 英文描述： | LC2MOS Complete 14-Bit DAC |
| 中文描述： | SERIAL, PARALLEL, WORD INPUT LOADING, 2.5 us SETTLING TIME, 14-BIT DAC, PDIP24 |
| 封裝： | PLASTIC, DIP-24 |
| 文件頁數： | 6/16頁 |
| 文件大小： | 337K |
| 代理商： | AD7840JN |
AD7840
REV. B
–6–
OP AMP SE CT ION
T he output from the voltage mode DAC is buffered by a
noninverting amplifier. Internal scaling resistors on the AD7840
configure an output voltage range of
±
3 V for an input reference
voltage of +3 V. T he arrangement of these resistors around the
output op amp is as shown in Figure 1. T he buffer amplifier is
capable of developing
±
3 V across a 2 k
and 100 pF load to
ground and can produce 6 V peak-to-peak sine wave signals to a
frequency of 20 kHz. T he output is updated on the falling edge
of the
LDAC
input. T he amplifier settles to within 1/2 LSB of
its final value in typically less than 2.5
μ
s.
T he small signal (200 mV p-p) bandwidth of the output buffer
amplifier is typically 1 MHz. T he output noise from the ampli-
fier is low with a figure of 30 nV/
√
Hz
at a frequency of 1 kHz.
T he broadband noise from the amplifier exhibits a typical peak-
to-peak figure of 150
μ
V for a 1 MHz output bandwidth. Figure
4 shows a typical plot of noise spectral density versus frequency
for the output buffer amplifier and for the on-chip reference.
Figure 4. Noise Spectral Density vs. Frequency
T RANSFE R FUNCT ION
T he basic circuit configuration for the AD7840 is shown in Fig-
ure 5. T able II shows the ideal input code to output voltage re-
lationship for this configuration. Input coding to the DAC is 2s
complement with 1 LSB = FS/16,384 = 6 V/16,384 = 366
μ
V.
Figure 5. AD7840 Basic Connection Diagram
T able II. Ideal Input/Output Code T able
DAC Latch Contents
MSB
LSB
Analog Output, V
OUT
*
0 1 1 1 1 1 1 1 1 1 1 1 1 1
0 1 1 1 1 1 1 1 1 1 1 1 1 0
0 0 0 0 0 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 0 0 0 0 0 0 0 0 0 0 0 0 1
1 0 0 0 0 0 0 0 0 0 0 0 0 0
+2.999634 V
+2.999268 V
+0.000366 V
0 V
–0.000366 V
–2.999634 V
–3 V
*Assuming REF IN = +3 V.
T he output voltage can be expressed in terms of the input code,
N, using the following expression:
V
OUT
=
2
×
N
×
REFIN
16384
8192
≤
N
≤ +
8191
INT E RFACE LOGIC INFORMAT ION
T he AD7840 contains two 14-bit latches, an input latch and a
DAC latch. Data can be loaded to the input latch in one of two
basic interface formats. T he first is a parallel 14-bit wide data
word; the second is a serial interface where 16 bits of data are
serially clocked into the input latch. In the parallel mode,
CS
and
WR
control the loading of data. When the serial data format
is selected, data is loaded using the SCLK ,
SYNC
and SDAT A
serial inputs. Data is transferred from the input latch to the
DAC latch under control of the
LDAC
signal. Only the data in
the DAC latch determines the analog output of the AD7840.
Parallel Data Format
T able III shows the truth table for AD7840 parallel mode op-
eration. T he AD7840 normally operates with a parallel input
data format. In this case, all 14 bits of data (appearing on data
inputs D13 (MSB) through D0 (LSB)) are loaded to the
AD7840 input latch at the same time.
CS
and
WR
control the
loading of this data. T hese control signals are level-triggered;
therefore, the input latch can be made transparent by holding
both signals at a logic low level. Input data is latched into the in-
put latch on the rising edge of
CS
or
WR
.
T he DAC latch is also level triggered. T he DAC output is nor-
mally updated on the falling edge of the
LDAC
signal. However,
both latches cannot become transparent at the same time.
T herefore, if
LDAC
is hardwired low, the part operates as fol-
lows; with
LDAC
low and
CS
and
WR
high, the DAC latch is
transparent. When
CS
and
WR
go low (with
LDAC
still low),
the input latch becomes transparent but the DAC latch is dis-
abled. When
CS
or
WR
return high, the input latch is locked
out and the DAC latch becomes transparent again and the DAC
output is updated. T he write cycle timing diagram for parallel
data is shown in Figure 6. Figure 7 shows the simplified parallel
input control logic for the AD7840.
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