AD781 Analog Devices, AD781 Datasheet - Page 7

• Manufacturer Part Number: AD781
• Description: Complete 700 ns Sample-and-Hold Amplifier
• Manufacturer: Analog Devices

REV. A
common pin should also be connected to the digital ground,
which is usually tied to analog common at the A-to-D converter.
Figure 4 illustrates the recommended decoupling and grounding
practice.
NOISE CHARACTERISTICS
Designers of data conversion circuits must also consider the
effect of noise sources on the accuracy of the data acquisition
system. A sample-and-hold amplifier that precedes the A-to-D
converter introduces some noise and represents another source
of uncertainty in the conversion process. The noise from the
AD781 is specified as the total output noise, which includes
both the sampled wideband noise of the SHA in addition to the
band limited output noise. The total output noise is the rms
sum of the sampled dc uncertainty and the hold mode noise. A
plot of the total output noise vs. the equivalent input bandwidth
of the converter being used is given in Figure 5.
DRIVING THE ANALOG INPUTS
For best performance, it is important to drive the AD781 analog
input from a low impedance signal source. This enhances the
sampling accuracy by minimizing the analog and digital
crosstalk. Signals which come from higher impedance sources
(e.g., over 5 k ) will have a relatively higher level of crosstalk.
For applications where signals have high source impedance, an
operational amplifier buffer in front of the AD781 is required.
The AD711 (precision BiFET op amp) is recommended for
these applications.
HIGH FREQUENCY SAMPLING
Aperture jitter and distortion are the primary factors which limit
frequency domain performance of a sample-and-hold amplifier.
Aperture jitter modulates the phase of the hold command and
produces an effective noise on the sampled analog input. The
magnitude of the jitter induced noise is directly related to the
frequency of the input signal.
A graph showing the magnitude of the jitter induced error vs.
frequency of the input signal is given in Figure 6.
The accuracy in sampling high frequency signals is also con-
strained by the distortion and noise created by the sample-and
hold. The level of distortion increases with frequency and re-
duces the “effective number of bits” of the conversion.
Figure 5. RMS Noise vs. Input Bandwidth of ADC
300
200
100
0
1k
10k
FREQUENCY – Hz
100k
1M
10M
–7–
Measurements of Figures 7 and 8 were made using a 14-bit A/D
converter with V
100 kSPS.
1/2 BIT @
1/2 BIT @
1/2 BIT @
1/2 BIT @
10 BITS
12 BITS
14 BITS
Figure 8. Signal/(Noise and Distortion) vs. Frequency
8 BITS
Figure 7. Total Harmonic Distortion vs. Frequency
90
80
70
60
50
40
30
20
10
–65
–70
–75
–80
–85
–90
–95
0
100
Figure 6. Error Magnitude vs. Frequency
0.01%
100
0.1%
1%
1k
IN
= 10 V p-p and a sample frequency of
1k
1k
FREQUENCY – Hz
FREQUENCY – Hz
APERTURE JITTER TYPICAL AT 50ps
10k
FREQUENCY – Hz
10k
10k
100k
100k
AD781
100k
1M
1M