AD7657BSTZ datasheet(15/25 Pages) AD

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Preliminary Technical Data

AD7658/AD7657/AD7656

Rev. PrI | Page 15 of 25

CONVERTER DETAILS

The AD7658/AD7657/AD7656 are high-speed, low power

converters that allow the simultaneous sampling of their six on-

chip ADCs. The Analog Inputs on the

AD7658/AD7657/AD7656 can accept True bipolar Input

signals, the RANGE pin/RNG bits are used to select between ±4

x VREF or ±2 x VREF as the Input Range for the next

conversion.

The AD7658/AD7657/AD7656 contain six SAR ADCs, six

track-and-hold amplifiers, on-chip 2.5V reference, reference

buffers, high speed parallel and serial interfaces. The

AD7658/AD7657/AD7656 allow the simultaneous sampling of

all six ADCs when all three CONVST signals are tied together.

Alternatively the six ADCs can be grouped into three pairs.

Each pair has an associated CONVST signal used to initiate

simultaneous sampling on each ADC pair, on four ADCs or all

six ADCs. CONVSTA is used to initiate simultaneous sampling

on V1 and V2, CONVSTB is used to initiate simultaneous

sampling on V3 and V4, and CONVSTC is used to initiate

simultaneous sampling on V5 and V6.

A conversion is initiated on the AD7658/AD7657/AD7656 by

pulsing the CONVSTX input. On the rising edge of CONVSTX

the track-and-hold on the selected ADCs will be placed into

hold mode and the conversions are started. After the rising edge

of CONVSTX the BUSY signal will go high to indicate the

conversion is taking place. The conversion clock for the part is

internally generated and the conversion time for the

AD7658/AD7657/AD7656 is 3 µs from the rising edge of

CONVSTX. The BUSY signal will return low to indicate the end

of conversion. On the falling edge of BUSY the track-and-hold

will return to track mode. Data can be read from the output

Track-and-Hold Section

The track-and-Hold amplifiers on the

AD7658/AD7657/AD7656 allow the ADCs to accurately

convert an input sine wave of full-scale amplitude to 12/14/16-

bit resolution. The input bandwidth of the track-and-hold

amplifiers is greater that the Nyquist rate of the ADC even

when the AD7658/AD7657/AD7656 is operating at its

maximum throughput rate. The AD7658/AD7657/AD7656 can

handle input frequencies up to 8 MHz.

The track-and-hold amplifiers sample their respective inputs

simultaneously on the rising edge of CONVSTX. The aperture

time for the track-and-hold, (i.e. the delay time between the

external CONVSTX signal actually going into hold), is typically

20ns. This is well matched across all six track-and-holds on the

one device and also from device to device. This allows more

than six ADCs to be simultaneously sampled. The end of the

conversion is signaled by the falling edge of BUSY and its at this

point the track-and-holds return to track mode and the

acquisition time begins.

Analog Input Section

The AD7658/AD7657/AD7656 can handle True bipolar input

voltages. The logic level on the RANGE pin or the value written

to the RNGX bits in the Control register will determine the

Analog input Range on the AD7658/AD7657/AD7656 for the

next conversion. When the RANGE pin/ RNGX bit is 1 the

Analog input range for the next conversion is ±2 x VREF, when

the RANGE pin/ RNG bit is 0 the Analog Input range for the

next conversion is ±4 x VREF.

VDD

VSS

Figure 3. Equivalent Analog Input Structure

Figure 3 shows an equivalent circuit of the analog input

structure of the AD7658/AD7657/AD7656. The two diodes, D1

and D2, provide ESD protection for the analog inputs. Care

must be taken to ensure that the analog input signal never

exceeds the VDD and VSS supply rails by more than TBD mV.

This will cause these diodes to become forward-biased and to

start conducting current into the substrate. The maximum

current these diodes can conduct without causing irreversible

damage to the part is 10 mA. Capacitor C1 in Figure 3 is

typically about 5 pF and can be attributed primarily to pin

capacitance. Resistor R1 is a lumped component made up of the

on resistance of a switch (track-and-hold switch). This resistor

is typically about 25 Ω. Capacitor C2 is the ADC sampling

capacitor and has a capacitance of 25 pF typically.

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