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參數資料
| 型號： | AD7895 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 14-Bit 50 kSPS ADC Ser. Out, 1.8V Operation 8-MSOP |
| 中文描述： | 5伏，12位，串行ADC的3.8毫秒的8引腳封裝 |
| 文件頁數： | 6/12頁 |
| 文件大小： | 314K |
| 代理商： | AD7895 |
AD7895
–6–
REV. 0
CONVERTER DETAILS
The AD7895 is a fast, 12-bit single supply A/D converter. It
provides the user with signal scaling, track/hold, A/D converter
and serial interface logic functions on a single chip. The A/D
converter section of the AD7895 consists of a conventional
successive-approximation converter based around an R-2R
ladder structure. The signal scaling on the AD7895-10 and
AD7895-3 allows the part to handle
±
10 V and
±
2.5 V input
signals, respectively, while operating from a single +5 V supply.
The AD7895-2 accepts an analog input range of 0 V to +2.5 V.
The part requires an external +2.5 V reference. The reference
input to the part is buffered on-chip.
The AD7895 has two
operating modes, the high sampling mode and the auto sleep
mode, where the part automatically goes into sleep after the end of
conversion. These modes are discussed in more detail in the
“Timing and Control” section.
A major advantage of the AD7895 is that it provides all of the
above functions in an 8-pin package, either 8-pin mini-DIP or
SOIC. This offers the user considerable spacing saving advantages
over alternative solutions. The AD7895 consumes only 20mW
maximum, making it ideal for battery-powered applications.
Conversion is initiated on the AD7895 by pulsing the
CONVST
input. On the falling edge of
CONVST
, the on-chip track/hold
goes from track to hold mode, and the conversion sequence is
started. The conversion clock for the part is generated internally
using a laser-trimmed clock oscillator circuit. Conversion time
for the AD7895 is 3.8
μ
s in the high sampling mode (9.8
μ
s for
the auto sleep mode), and the track/hold acquisition time is
0.3
μ
s. To obtain optimum performance from the part, the read
operation should not occur during the conversion or during
300 ns prior to the next conversion. This allows the part to
operate at throughput rates up to 192 kHz and achieve data sheet
specifications.
CIRCUIT DESCRIPTION
Analog Input Section
The AD7895 is offered as three part types: the AD7895-10,
which handles a
±
10 V input voltage range; the AD7895-3,
which handles input voltage range
±
2.5 V; and the AD7895-2,
which handles a 0 V to +2.5 V input voltage range.
AGND
AD7895-10/AD7895-3
V
IN
REF IN
TRACK/
HOLD
TO ADC
REFERENCE
CIRCUITRY
TO INTERNAL
COMPARATOR
R3
R2
R1
Figure 2. AD7895-10/AD7895-3 Analog Input Structure
Figure 2 shows the analog input section for the AD7895-10 and
AD7895-3. The analog input range of the AD7895-10 is
±
10 V
into an input resistance of typically 33 k
. The analog input
range of the AD7895-3 is
±
2.5 V into an input resistance of
typically 12 k
.
This input is benign with no dynamic charging
currents, as the resistor stage is followed by a high input imped-
ance stage of the track/hold amplifier. For the AD7895-10,
R1 = 30 k
, R2 = 7.5 k
and R3 = 10 k
. For the AD7895-3,
R1 = R2 = 6.5 k
and R3 is open circuit.
For the AD7895-10 and AD7895-3, the designed code transi-
tions occur on successive integer LSB values (i.e., 1 LSB, 2 LSBs,
3 LSBs . . .). Output coding is 2s complement binary with 1 LSB
= FS/4096. The ideal input/output transfer function for the
AD7895-10 and AD7895-3 is shown in Table I.
Table I. Ideal Input/Output Code Table for the AD7895-10/-3
Digital Output
Analog Input
l
Code Transition
+FSR/2 – 1 LSB
2
+FSR/2 – 2 LSBs
+FSR/2 – 3 LSBs
011 . . . 110 to 011 . . . 111
011 . . . 101 to 011 . . . 110
011 . . . 100 to 011 . . . 101
GND + 1 LSB
GND
GND – 1 LSB
000 . . . 000 to 000 . . . 001
111 . . . 111 to 000 . . . 000
111 . . . 110 to 111 . . . 111
–FSR/2 + 3 LSBs
–FSR/2 + 2 LSBs
–FSR/2 + 1 LSB
100 . . . 010 to 100 . . . 011
100 . . . 001 to 100 . . . 010
100 . . . 000 to 100 . . . 001
NOTES
1
FSR is full-scale range = 20 V (AD7895-10) and = 5 V (AD7895-3)
with REF IN = +2.5 V.
2
1 LSB = FSR/4096 = 4.883 mV (AD7895-10) and 1.22 mV (AD7895-3)
with REF IN = +2.5 V.
The analog input section for the AD7895-2 contains no biasing
resistors, and the V
IN
pin drives the input to the track/hold
amplifier directly. The analog input range is 0 V to +2.5 V into
a high impedance stage with an input current of less than
500 nA. This input is benign with no dynamic charging cur-
rents. Once again, the designed code transitions occur on succes-
sive integer LSB values. Output coding is straight (natural) binary
with 1 LSB = FS/4096 = 2.5 V/4096 = 0.61 mV. Table II shows
the ideal input/output transfer function for the AD7895-2.
Table II. Ideal Input/Output Code Table for AD7895-2
Digital Output
Code Transition
Analog Input
1
+FSR – 1 LSB
2
+FSR – 2 LSB
+FSR – 3 LSB
111 . . . 110 to 111 . . . 111
111 . . . 101 to 111 . . . 110
111 . . . 100 to 111 . . . 101
GND + 3 LSB
GND + 2 LSB
GND + 1 LSB
000 . . . 010 to 000 . . . 011
000 . . . 001 to 000 . . . 010
000 . . . 000 to 000 . . . 001
NOTES
1
FSR is full-scale range and is 2.5 V for AD7895-2 with VREF = +2.5 V.
2
1 LSB = FSR/4096 and is 0.61 mV for AD7895-2 with VREF = +2.5 V.
Track/Hold Section
The track/hold amplifier on the analog input of the AD7895
allows the ADC to accurately convert an input sine wave of full-
scale amplitude to 12-bit accuracy. The input bandwidth of the
track/hold is greater than the Nyquist rate of the ADC even
when the ADC is operated at its maximum throughput rate of
192 kHz (i.e., the track/hold can handle input frequencies in
excess of 100 kHz).
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