AD7656ABSTZ datasheet(19/29 Pages) AD

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AD7656A

Data Sheet

CONVST A,

CONVST B,

CONVST C

BUSY

DATA

tQUIET

tACQ

tCONVERT

t10

Figure 23. Parallel Interface Timing Diagram (W/B = 0)

LOW BYTE

HIGH BYTE

DB15 TO DB8

Figure 24. Parallel Interface—Read Cycle for Byte Mode of Operation (W/B = 1, HBEN = 0)

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 23). 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.

When using the three CONVST x signals to independently initiate

conversions on the three ADC pairs, 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, tie the associated CONVST x pin to VDRIVE.

If there is only an 8-bit bus available, the AD7656A 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 AD7656A

can be accessed in two read operations, with eight bits of data

provided on DB15 to DB8 for each of the read operations (see

Figure 24). 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, tie the HBEN pin low. To always access the high

byte first on DB15 to DB8, tie the HBEN pin high. In byte mode

when all three CONVST x pins are pulsed together to initiate

simultaneous conversions on all six ADCs, 12 read operations

are necessary to read back the six 16-bit conversion results.

Leave DB6 to DB0 unconnected in byte mode.

Rev. 0 | Page 18 of 28

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