10

®

SHC605

DISCUSSION OF

PERFORMANCE

DISTORTION

Hold-mode distortion is an important specification for a

track-and-hold amplifier. This is a measure of the accuracy

of the amplifier’s held output while sampling a sinusoidal

input signal. It includes errors from both the switching

network and the amplifier’s signal path. Hold-mode distor-

tion depends on the input signal’s amplitude and frequency

as well as the sampling rate. The biggest cause of distortion

in the SHC605 is slew-induced nonlinearity; the higher the

amplitude of a high frequency input, the higher the distor-

tion. Hold-mode distortion can also result from sampling too

fast or not allowing enough acquisition time or track-to-hold

settling time. The SHC605 has a typical 0.01% acquisition

time of 30ns for a 2V step, and a typical 100

µV track-to-

hold settling time of 15ns. Thus, for 12-bit accuracy the

clock rate should not exceed 22MHz (refer to Typical

Performance Curves for details).

NOISE

The SHC605’s noise performance is almost completely

determined by track-mode noise. This is the noise sampled

by the differential hold capacitors during track-mode, which

is greater than the noise measured directly at the output. The

input referred noise of the SHC605 is 2.5nV/

√Hz. For unity-

gain this corresponds to an output noise of approximately

35

µVrms; which is much lower than the typical 150µVrms

noise sampled by the hold capacitors. The track-mode noise

sampled by the hold capacitors is independent of closed-

loop gain, and therefore, the SHC605 can be used with

higher closed-loop gain without degrading the overall noise

performance.

The SHC605’s noise performance is also affected by hold-

mode noise and aperture jitter. Hold-mode noise is the result

of current noise reacting with the hold capacitors. This noise

accumulates on the capacitors at a rate which is proportional

to the square root of the hold time. For sample rates above

1MHz this noise is usually insignificant. Aperture jitter

describes the random variation in track-to-hold aperture

delay, and causes increased hold-mode noise when high

slew rate signals are sampled. A differential ECL clock input

will provide lower aperture jitter than a single-ended ECL or

TTL clock.

CHOOSING THE BEST ARCHITECTURE

The SHC605 is basically a high-speed operational amplifier

which can hold its output on command. Unlike traditional

high-speed track-and-hold amplifiers, which have fixed gains

of +1V/V, the SHC605 can be used with non-inverting,

inverting, or differential gains. In many applications, a

single SHC605 can be used to solve a problem that previ-

ously required two or more devices.

Figures 3 through 6 show the SHC605 connected for non-

inverting, inverting, and differential gains. As with any op

amp, it is important to consider performance tradeoffs for all

of these configurations. For gains less than

±10, the SHC605’s

track-to-hold settling, pedestal offset, droop, and total hold-

mode noise remains constant. However, small-signal band-

width and acquisition time will be compromised as the

closed-loop gain is increased (refer to the Typical Perfor-

mance Curves for details).

FIGURE 3. Gain of +1 Track-and-Hold Amplifier.

FIGURE 4. Gain of +2 Track-and-Hold Amplifier.

+In

+V

S

+V

S

DGND

AGND

AGND

AGND

–In

–V

S

–V

S

Lock

Lock

Hold

Hold

Select

V

OUT

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

1µF

.01µF

.01µF

1µF

–5V

+5V

V

IN

NC

Hold

+5V = TTL

–5V = ECL

V

OUT

50

Ω

+In

+V

S

+V

S

DGND

AGND

AGND

AGND

–In

–V

S

–V

S

Lock

Lock

Hold

Hold

Select

V

OUT

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

1µF

.01µF

.01µF

1µF

–5V

+5V

V

IN

NC

Hold

+5V = TTL

–5V = ECL

V

OUT

249

Ω

249

Ω

R

1

R

2

V

IN

V

OUT

R

1

R

2

= 1 +