AD7658BSTZ datasheet(23/32 Pages) AD

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Data Sheet

AD7656/AD7657/AD7658

Rev. D | Page 23 of 32

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 Figure 27). If CONVST A

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

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 VDRIVE.

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 Figure 28). The HBEN pin

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.

CONVST A,

CONVST B,

CONVST C

BUSY

DB0 TO DB15

tQUIET

tACQ

tCONVERT

t10

Figure 27. Parallel Interface Timing Diagram (W/B = 0)

LOW BYTE

HIGH BYTE

DB15 TO DB8

Figure 28. Parallel Interface—Read Cycle for Byte Mode of Operation (W/B = 1, HBEN = 0)

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