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參數資料
| 型號： | AD7653 |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | 圓形連接器 |
| 英文描述： | Circular Connector; No. of Contacts:4; Series:D38999; Body Material:Metal; Connecting Termination:Crimp; Connector Shell Size:13; Circular Contact Gender:Socket; Circular Shell Style:Straight Plug; Insert Arrangement:13-4 |
| 中文描述： | 16位1 MSPS的特區(qū)單極型ADC檔 |
| 文件頁數： | 15/23頁 |
| 文件大?。?/td> | 336K |
| 代理商： | AD7653 |
REV. PrA
PRELIMINARY TECHNICAL DATA
AD7667
–15–
T he AD829 is another alternative where high-frequency
(above 100 kHz) performance is not required. In gain of
1, it requires an 82 pF compensation capacitor.
T he AD8610 is another option where low bias current is
needed in low-frequency applications.
Voltage R eference Input
T he AD7667 allows the choice of either an internal 2.5 V
voltage reference or an external 2.5 V reference.
T o use the internal reference along with the internal buffer,
PDREF and PDBUF should both be LOW. T his will pro-
duce a voltage on REFBUFIN of 1.25 V and the buffer’s
gain will be 2, resulting in a 2.5 V reference on REF pin.
T o use an external reference along with the internal
buffer, PDREF should be HIGH and PDBUF should be
LOW. T his powers down the internal reference and allows
for the 2.5 V reference to be applied to REFBUFIN. In
this mode the buffer’s gain is 1.
T o use both external reference, PDREF and PDBUF
should both be HIGH. T he reference input should be
applied to REF.
It is useful to decouple the REFBUFIN pin with a 100
nF
ceramic capacitor.
T he output impedance of the REFBUFIN
pin is 4 k
. T hus, the 100
nF capacitor provides an RC filter
for noise reduction.
It should be noted that the internal reference and internal
buffer are independent of the power down (PD) pin of the
part. Powering down the part does not power down the inter-
nal reference or the internal buffer. Furthermore, powering
down the internal reference and internal buffer, as well as
powering them up, requires time. T his is due to the fact that
we have charging and discharging capacitors on the REF
which require some settling time. T herefore, for applications
requiring low power, there will always be a typical of 10 mW
of power dissipated when using the internal reference and
internal buffer even during times with no conversions.
T he internal reference is temperature compensated to
2.5V ± T BD mV.
T he reference is trimmed to provide
a typical drift of T BD ppm/ C . T his typical drift char-
acteristic is shown in Figure T BD. For improved drift
performance, an external reference such as the AD780
can be used
.
-160
-140
-120
-100
-80
-60
-40
-20
0
1
TO BE
SUPPLIED
Figure T BD
For the external reference, the voltage reference input REF
of the AD7667 has a dynamic input impedance; it should
therefore be driven by a low-impedance source with an
efficient decoupling between REF and REFGND inputs.
T his decoupling depends on the choice
of the voltage refer-
ence but usually consists of a 1 μF
ceramic capacitor and a
low ESR tantalum capacitor connected to the REF and
REFGND inputs with minimum parasitic inductance. 47
μF is an appropriate value for the tantalum capacitor when
using either the internal reference of one of the recom-
mended reference voltages:
T he low noise, low temperature drift ADR421 and
AD780 voltage references
T he low power ADR291 voltage reference
T he low cost AD1582 voltage reference
For applications using multiple AD7667s, it is more effective
to buffer the reference voltage using the internal buffer. T o
do so, PDREF should be HIGH, and PDBUF should be low.
Care should also be taken with the reference temperature
coefficient of the voltage reference which directly affects the
full-scale accuracy if this parameter matters. For instance, a
±15 ppm/°C tempco of the reference changes the full scale by
±1 LSB/°C.
V
REF
, as mentioned in the specification table, could be increased
to AVDD – 1.85 V. T he benefit here is the increased SNR
obtained as a result of this increase. Since the input range is
defined in terms of V
REF
, this would essentially increase the
range to make it a 0 to 3 V input range with an AVDD above
4.85 V. One of the benefits here is the additional SNR ob-
tained as a result of this increase. T he theoretical
improvement as a result of this increase in reference is 1.58
dB (20 log [3/2.5]). Due to the theoretical quantization
noise, however, the observed improvement is approximately
1 dB. T he AD780 can be selected with a 3 V reference
voltage.
T he T EMP pin, which measures the temperature of the
AD7667, can be used as follows. Refer to figure T BD to
see the connectivity. T he output of the T EMP pin is ap-
plied to one of the inputs of the analog switch (ADG779).
T he other input, as shown is the analog signal. T he output
of the switch is connected to the AD8021 which is config-
ured as a follower. T he output of the op-amp is applied to
the IN pin. Refer to the Specification T able for the appro-
priate values related to the T EMP pin. T his configuration
could be very useful to improve the calibration accuracy
over the temperature range.
C
C
AD8021
IN
AD7667
ANALOG INPUT
(UNIPOLAR)
IN
temperature
sensor
ADG779
TEMP
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