SHC605 Datasheet, PDF (10/15 Pages) Burr-Brown (TI) – High-Speed Operational TRACK-AND-HOLD AMPLIFIER

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 100µV 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
35µVrms; 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).
V_IN
+5V
1µF
.01µF
1 +In
2 +V_S
3 +V_S
4 DGND
–V_S 16
–V_S 15
Lock 14
Lock 13
5 AGND Hold 12
6 AGND Hold 11 NC
7 AGND Select 10
8 –In
V_OUT 9
.01µF
–5V
1µF
Hold
+5V = TTL
–5V = ECL
V_OUT
50Ω
FIGURE 3. Gain of +1 Track-and-Hold Amplifier.
V_IN
+5V
1µF
.01µF
1 +In
2 +V_S
3 +V_S
4 DGND
–V_S 16
–V_S 15
Lock 14
Lock 13
5 AGND Hold 12
6 AGND Hold 11 NC
7 AGND Select 10
8 –In
V_OUT 9
.01µF
–5V
1µF
Hold
+5V = TTL
–5V = ECL
V_OUT
R₁
R₂ 249Ω
249Ω
V_{OUT = 1 +} R₁
V_IN
R₂
FIGURE 4. Gain of +2 Track-and-Hold Amplifier.
®
SHC605
10