AD684

REV. A

–5–

DEFINITIONS OF SPECIFICATIONS

Acquisition Time

— The length of time that the SHA must

remain in the sample mode in order to acquire a full-scale input

step to a given level of accuracy.

Small Signal Bandwidth

— The frequency at which the held

output amplitude is 3 dB below the input amplitude, under an

input condition of a 100 mV p-p sine wave.

Full Power Bandwidth

— The frequency at which the held

output amplitude is 3 dB below the input amplitude, under an

input condition of a 10 V p-p sine wave.

Effective Aperture Delay

— The difference between the

switch delay and the analog delay of the SHA channel. A

negative number indicates that the analog portion of the overall

delay is greater than the switch portion. This effective delay

represents the point in time, relative to the hold command, that

the input signal will be sampled.

Aperture Jitter

— The variations in aperture delay for

successive samples. Aperture jitter puts an upper limit on the

maximum frequency that can be accurately sampled.

Hold Settling Time

—The time required for the output to

settle to within a specified level of accuracy of its final held

value after the hold command has been given.

Droop Rate

— The drift in output voltage while in the hold

mode.

Feedthrough

— The attenuated version of a changing input

signal that appears at the output when the SHA is in the hold

mode.

Hold Mode Offset

— The difference between the input signal

and the held output. This offset term applies only in the hold

mode and includes the error caused by charge injection and all

other internal offsets. It is specified for an input of 0 V.

Tracking Mode Offset

— The difference between the input

and output signals when the SHA is in the track mode.

Nonlinearity

— The deviation from a straight line on a plot of

input vs. (held) output as referenced to a straight line drawn

between endpoints, over an input range of –5 V and +5 V.

Gain Error

— Deviation from a gain of +1 on the transfer

function of input vs. held output.

Interchannel Isolation

— The level of crosstalk between

adjacent channels while in the sample (track) mode with a full

scale 100 kHz input signal.

Interchannel Aperture Offset

— The variation in aperture

time between the four channels for a simultaneous hold

command.

Differential Offset

— The difference in hold mode offset

between the four SHA channels.

Power Supply Rejection Ratio

— A measure of change in the

held output voltage for a specified change in the positive or

negative supply.

Sampled dc Uncertainty

— The internal rms SHA noise that

is sampled onto the hold capacitor.

Hold Mode Noise

— The rms noise at the output of the SHA

while in the hold mode, specified over a given bandwidth.

Total Output Noise

— The total rms noise that is seen at the

output of the SHA while in the hold mode. It is the rms

summation of the sampled dc uncertainty and the hold mode

noise.

Output Drive Current

— The maximum current the SHA can

source (or sink) while maintaining a change in hold mode offset

of less than 2.5 mV.

FUNCTIONAL DESCRIPTION

The AD684 is a complete quad sample-and-hold amplifier that

provides high speed sampling to 12-bit accuracy in less than 1

µs.

The AD684 is completely self-contained, including on-chip

hold capacitors, and requires no external components or

adjustments to perform the sampling function. Each SHA

channel can operate independently, having its own input, output

and sample/hold command. Both inputs and outputs are treated

as single ended signals, referred to common.

The AD684 utilizes a proprietary circuit design which includes a

self-correcting architecture. This sample-and-hold circuit

corrects for internal errors after the hold command has been

given, by compensating for amplifier gain and offset errors, and

charge injection errors. Due to the nature of the design, the

SHA output in the sample mode is not intended to provide an

accurate representation of the input. However, in hold mode,

the internal circuitry is reconfigured to produce an accurately

held version of the input signal. To the right is a block diagram

of the AD684.

Functional Block Diagram