SHC5320 Datasheet, PDF (8/10 Pages) Burr-Brown (TI) – High-Speed, Bipolar, Monolithic SAMPLE/HOLD AMPLIFIER

diodes may be Schottky diodes, which will provide the
fastest clamping action and lowest clamping voltage, but
fast signal diodes such as IN914 will also work in most
applications. In each configuration the value of R₁ should be
large enough to avoid excessive loading of the input signal
source.
Similarly,
R
2
should
have
a
value
of
2kΩ
or
greater
to insure sufficient load current capability from the sample/
hold. If the value of R becomes too large, however, the
2
added capacitance of the diodes may change the sample/hold
phase response enough to cause oscillation.
R₁
Input
R₂
1
2
7
Output
FIGURE 7. Input Overload Protection—Inverting Configu-
ration.
R₂
1
R₁
2
Input
7
Output
FIGURE 8. Input Overload Protection—Noninverting Con-
figuration.
APPLICATIONS
(Developed Around 14-Pin Package)
SIGNAL DIGITIZATION
Sample/hold amplifiers are normally used to hold input
voltages to an A/D converter constant during conversion.
Digitizing errors result if the analog signal being digitized
varies excessively during conversion.
For example, the Burr-Brown ADC80MAH-12 is a 12-bit
successive-approximation converter with a 25µs conversion
time. To insure the accuracy of the output data, the analog
input signal to the A/D converter must not change more than
1/2LSB during conversion.
The maximum rate of change of a sine wave of frequency,
f, is dv/dt (max) = 2πAf(V/s). If one allows a 1/2LSB change
(2.44mV) for a ±10V input swing to the A/D converter, the
allowable input rate-of-change limit would be 2.44mV/25µs
= 0.0976mV/µs. Thus the sampled sinusoidal signal fre-
quency limit is
f = (0.0976 x 10³)/2πA = 15.5/A (Hz),
where A is the peak amplitude of the sine wave. For a ±10V
sine wave, this corresponds to a frequency of 1.6Hz, hardly
acceptable for the majority of sampled data systems.
However, a sample/hold in front of the A/D converter
“freezes” the converter’s input signal whenever it is neces-
sary to make a conversion. The rate-of-change limitation
calculated above no longer exists. If a sample/hold has
acquired an input signal and is tracking it, the sample/hold
can be commanded to hold it at any instant in time. There is
a short delay (aperture delay) between the time the hold
command is asserted and the time the circuit actually holds.
The hold command signal can usually be advanced in time
(or delayed, in the case of negative effective aperture delay)
to cause the amplifier to hold the signal actually desired.
Aperture uncertainty (also called aperture jitter) is also a key
consideration. For the SHC5320 there is a 300ps period
during which the signal should not change more than the
amount allowed for aperture uncertainty in the system error
budget, perhaps 1/2LSB for a 12-bit system. For a ±10V
input range (1/2LSB = 2.44mV), the input signal rate of
change limitation is 2.44mV/0.3ns = 8.13mV/ns. The equiva-
lent input sine wave frequency is
f = 8.13 X 10⁶/2πA = 1.29/A (MHz),
a factor of almost 84,000 higher than using the A/D alone.
However, there are other considerations. The resampling
rate of an ADC80/SHC5320 combination is 26.5µs (25µs
A/D) conversion time plus 1.5µs S/H acquisition time).
Sampling a sine wave at the Nyquist rate, this permits a
maximum input signal frequency of 37.7kHz. The above
analysis assumes that the droop rate of the sample/hold is
negligible—less than 1/2LSB during the conversion time—
and that the large signal bandwidth response of the sample/
hold causes negligible waveform distortion. Both of these
assumptions are valid for the SHC5320 in this application.
DATA ACQUISITION
The SHC5320 may be used to hold data for analog-to-digital
conversion or may be used to provide pulse-amplitude
modulation (PAM) data output (see Figures 9 and 10).
A/D
Converter
Analog
Inputs
1
7
2 ^SHC5320
PAM Output
14
6
Analog
Multiplexer
(Burr-Brown MPC Series)
Mode Signal
Control Common
FIGURE 9. Typical Data Acquisition Configuration.
®
SHC5320
8