AD7657 Analog Devices, AD7657 Datasheet - Page 23

• Manufacturer Part Number: AD7657
• Description: 250 kSPS, 6-Channel, Simultaneous Sampling, Bipolar, 14-Bit A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7657

• Resolution (bits): 14bit
• # Chan: 6
• Sample Rate: 250kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: SE-Bip
• Ain Range: Bip (Vref) x 2,Bip (Vref) x 4,Bip 10V,Bip 5.0V
• Adc Architecture: SAR
• Pkg Type: QFP

The CS signal can be permanently tied low, and the RD signal
can be used to access the conversion results. A read operation
can take place after the BUSY signal goes low. The number of
required read operations depends on the number of ADCs that
are simultaneously sampled (see
and CONVST B are simultaneously brought low, four read
operations are required to obtain the conversion results from
V1, V2, V3, and V4. If CONVST A and CONVST C are
simultaneously brought low, four read operations are required
to obtain the conversion results from V1, V2, V5, and V6.
The conversion results are output in ascending order. For
the AD7657, DB15 and DB14 contain two leading zeros and
DB[13:0] output the 14-bit conversion result. For the AD7658,
DB[15:12] contain four leading zeros, and DB[11:0] output the
12-bit conversion result.
When using the three CONVST signals to independently
initiate conversions on the three ADC pairs, care should be
taken to ensure that a conversion is not initiated on a channel
pair when the BUSY signal is high. It is also recommended not
to initiate a conversion during a read sequence because doing so
DB0 TO DB15
CONVST A,
CONVST B,
CONVST C
BUSY
RD
CS
t
CONVERT
DB15 TO DB8
CS
RD
Figure 28. Parallel Interface—Read Cycle for Byte Mode of Operation ( W /B = 1, HBEN = 0)
Figure 27
t
2
t
3
). If CONVST A
V1
Figure 27. Parallel Interface Timing Diagram ( W /B = 0)
t
4
t
3
t
t
6
V2
5
LOW BYTE
t
5
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t
6
t
t
7
t
8
4
V3
can affect the performance of the conversion. For the specified
performance, it is recommended to perform the read after the
conversion. For unused input channel pairs, the associated
CONVSTx pin should be tied to V
If there is only an 8-bit bus available, the AD7656/AD7657/
AD7658 interface can be configured to operate in byte mode
( W /B = 1). In this configuration, the DB7/HBEN/DCEN pin
takes on its HBEN function. Each channel conversion result
from the AD7656/AD7657/AD7658 can be accessed in two
read operations, with 8 bits of data provided on DB15 to DB8
for each of the read operations (see
determines whether the read operation first accesses the high
byte or the low byte of the 16-bit conversion result. To always
access the low byte first on DB15 to DB8, the HBEN pin should
be tied low. To always access the high byte first on DB15 to
DB8, the HBEN pin should be tied high. In byte mode when all
three CONVST pins are pulsed together to initiate simultaneous
conversions on all six ADCs, 12 read operations are necessary
to read back the six 16-/14-/12-bit conversion results. DB[6:0]
should be left unconnected in byte mode.
t
9
t
t
9
ACQ
V4
t
7
V5
HIGH BYTE
t
8
AD7656/AD7657/AD7658
V6
DRIVE
Figure 28
.
t
QUIET
). The HBEN pin
t
10