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參數(shù)資料
| 型號： | AD7865 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Simultaneous Sampling, Fast, 14-Bit ADC(四通道,同時采樣,高速,14位A/D轉(zhuǎn)換器) |
| 中文描述： | 同步采樣，快速，14位ADC（四通道，同時采樣，高速，14位的A / D轉(zhuǎn)換器） |
| 文件頁數(shù)： | 9/21頁 |
| 文件大?。?/td> | 375K |
| 代理商： | AD7865 |
AD7865
Rev. PrC
–9–
Preliminary Technical Data
The conversion clock for the part is generated internally
using a laser-trimmed clock oscillator circuit. There is
also the option of using an external clock, by tying the
INT
/EXT CLK pin logic high, and applying an external
clock to the CLKIN pin. However the optimum through-
put is obtained by using the internally generated clock -
see Using an External Clock. The
EOC
signal indicates
the end of each conversion in the conversion sequence.
The BUSY signal indicates the end of the full conversion
sequence and at this time all four Track and Holds return
to tracking mode. The conversion results can either be
read at the end of the full conversion sequence (indicated
by BUSY going low) or as each result becomes available
(indicated by
EOC
going low). Data is read from the part
via a 14-bit parallel data bus with standard
CS
and
RD
signals - see Timing and Control.
Conversion time for each channel of the the AD7865 is
2.5μs and the track/hold acquisition time is 0.35 μs. To
obtain optimum performance from the part, the read
operation should not occur during a channel conversion or
during the 100 ns prior to the next
CONVST
rising edge.
This allows the part to operate at throughput rates up to
100 kHz for all four channels and achieve data sheet
specifications.
Track/Hold Section
The track/hold amplifier's on the AD7865 allows the
ADC's to accurately convert an input sine wave of full-
scale amplitude to 14-bit accuracy. The input bandwidth
TECHNCAL
CONVERTER DETAILS
The AD7865 is a high speed, low power, four-channel
simultaneous sampling 14-bit A/D converter that operates
from a single +5 V supply. The part contains a 2.5 μs
successive approximation ADC, four track/hold ampli-
fiers, an internal +2.5 V reference and a high speed
parallel interface. There are 4 analog inputs which can be
sampled simultaneously thus preserving the relative phase
information of the signals on all four analog inputs.
Thereafter, conversions will be completed on the selected
subset of the four channels. The part accepts an analog
input range of ±10V or ±5V (AD7865-1) or ±2.5V
(AD7865-3). Overvoltage protection on the analog inputs
for the part allows the input voltage to go to ±20 V , -5V
to +20V respectively without causing damage.
The AD7865
has two operating modes Reading Between Conversions and Reading
after the Conversion Sequence. These modes are discussed in more
detail in the Timing and Control Section.
A conversion is initiated on the AD7865 by pulsing the
CONVST
input. On the rising edge of
CONVST
, all
four on-chip track/holds are placed into hold simulta-
neously and the conversion sequence is started on all the
selected channels. Channel selection is made via the SL1 -
SL4 pins if
H
/S SEL is logic zero or via the channel
select register if
H
/S SEL is logic one - see Selecting a
Conversion Sequence. The channel select register is
programmed with via the bidirectional data lines DB0 -
DB3 and a standard write operation. The selected conver-
sion sequence is latched on the rising edge of
CONVST
so changing a selection will only take effect once a new
conversion sequence is initiated. The BUSY output signal
is triggered high on the rising edge of
CONVST
and will
remain high for the duration of the conversion sequence.
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 350kSPS (i.e., the track/hold can
handle input frequencies in excess of 175kHz).
The track/hold amplifier's acquire input signals to 14-bit
accuracy in less than 350ns. The operation of the track/
hold's are essentially transparent to the user. The four
track/hold amplifiers sample their respective input chan-
nels simultaneously, on the rising edge of
CONVST
.
The aperture time for the track/hold's (i.e., the delay time
between the external
CONVST
signal and the track/hold
actually going into hold) are typically 15 ns and, more
importantly, is well matched across the four track/holds on
one device and also well matched from device to device.
This allows the relative phase information between
different input channels to be accurately preserved. It also
allows multiple AD7865s to sample more than four
channels simultaneously. At the end of a conversion
sequence, the part returns to its tracking mode. The
acquisition time of the track/hold amplifier's begin at this
point.
Reference Section
The AD7865 contains a single reference pin, labelled
V
REF
,
which either provides access to the part’s own +2.5 V
reference or allows an external +2.5 V reference to be
connected to provide the reference source for the part.
The part is specified with a +2.5 V reference voltage.
Errors in the reference source will result in gain errors in
the AD7865’s transfer function and will add to the speci-
fied full-scale errors on the part. It will also result in an
offset error injected in the attenuator stage.
The AD7865 contains an on-chip +2.5 V reference. To
use this reference as the reference source for the AD7865,
simply connect a 0.1 μF disc ceramic capacitor from the
V
pin to AGND. The voltage that appears at this pin is
internally buffered before being applied to the ADC. If
this reference is required for use external to the AD7865,
it should be buffered as the part has a FET switch in se-
ries with the reference output resulting in a source imped-
ance for this output of 6k
nominal. The tolerance on the
internal reference is ±10 mV at 25°C with a typical tem-
perature coefficient of 25 ppm/°C and a maximum error
over temperature of ±20mV.
If the application requires a reference with a tighter
tolerance or the AD7865 needs to be used with a system
reference, then the user has the option of connecting an
external reference to this
V
pin. The external reference
will effectively overdrive the internal reference and thus
provide the reference source for the ADC. The reference
input is buffered before being applied to the ADC with the
maximum input current of ±100 μA. Suitable reference
sources for the AD7865 include the AD680, AD780,
REF192 and REF43 precision +2.5 V references.
CIRCUIT DESCRIPTION
Analog Input Section
The AD7865 is offered as two part types, the AD7865-1
where each input can be configured for ±10 V or a ±5 V
input voltage range and the AD7865-3 which handles
input voltage range ±2.5 V. The amount of current
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