Data Sheet

AD7608

Rev. A | Page 23 of 32

CONVERSION CONTROL

Simultaneous Sampling on All Analog Input Channels

The AD7608 allows simultaneous sampling of all analog input

channels. All channels are sampled simultaneously when both

CONVST x pins (CONVST A, CONVST B) are tied together.

A single CONVST x signal is used to control both CONVST x

inputs. The rising edge of this common CONVST x signal

initiates simultaneous sampling on all analog input channels.

The AD7608 contains an on-chip oscillator that is used to

perform the conversions. The conversion time for all ADC

conversions are in progress, so when the rising edge of CONVST x

is applied, BUSY goes logic high and transitions low at the end

of the entire conversion process. The falling edge of the BUSY

signal is used to place all eight track-and-hold amplifiers back

into track mode. The falling edge of BUSY also indicates that

the new data can now be read from the parallel bus (DB[15:0]),

Simultaneously Sampling Two Sets of Channels

The AD7608 also allows the analog input channels to be

sampled simultaneously in two sets. This can be used in power-

line protection and measurement systems to compensate for

phase differences introduced by PT and CT transformers. In a

50 Hz system, this allows for up to 9° of phase compensation; and

in a 60 Hz system, it allows for up to 10° of phase compensation.

This is accomplished by pulsing the two CONVST x pins

independently and is possible only if oversampling is not in

use. CONVST A is used to initiate simultaneous sampling of

the first set of channels (V1 to V4) and CONVST B is used

to initiate simultaneous sampling on the second set of analog

input channels (V5 to V8), as illustrated in Figure 41. On the

rising edge of CONVST A, the track-and-hold amplifiers for

the first set of channels are placed into hold mode. On the

rising edge of CONVST B, the track-and-hold amplifiers for

the second set of channels are placed into hold mode. The con-

version process begins once both rising edges of CONVST x

have occurred; therefore BUSY goes high on the rising edge of

maximum allowable time between CONVST x sampling points.

There is no change to the data read process when using two

separate CONVST x signals.

Connect all unused analog input channels to AGND. The results

for any unused channels are still included in the data read because

all channels are always converted.

CONVST A

CONVST B

BUSY

CS, RD

DATA: DB[15:0]

FRSTDATA

V1 TO V4 TRACK-AND-HOLD

ENTER HOLD

V5 TO V8 TRACK-AND-HOLD

ENTER HOLD

AD7608 CONVERTS

ON ALL 8 CHANNELS

V1

V8

V2

Figure 41. Simultaneous Sampling on Channel Sets Using Independent CONVST A/CONVST B Signals—Parallel Mode