AD783AR Analog Devices Inc, AD783AR Datasheet - Page 7

• Manufacturer Part Number: AD783AR
• Manufacturer: Analog Devices Inc

Specifications of AD783AR

• Number Of Sample And Hold Elements: 1
• Power Supply Requirement: Dual
• Single Supply Voltage (typ): Not RequiredV
• Single Supply Voltage (min): Not RequiredV
• Single Supply Voltage (max): Not RequiredV
• Lead Free Status / RoHS Status: Not Compliant

REV. A
The AD783 does not provide separate analog and digital ground
leads as is the case with most A-to-D converters. The common
pin is the single ground terminal for the device. It is the refer-
ence point for the sampled input voltage and the held output
voltage and also the digital ground return path. The common
pin should be connected to the reference (analog) ground of the
A-to-D converter with a separate ground lead. Since the analog
and digital grounds in the AD783 are connected internally, the
common pin should also be connected to the digital ground,
which is usually tied to analog common at the A-to-D converter.
Figure 3 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
AD783 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 4.
DRIVING THE ANALOG INPUTS
For best performance, it is important to drive the AD783 analog
input from a low impedance signal source. This enhances the
sampling accuracy by minimizing the analog and digital cross-
talk. 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 AD783 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 5.
Figure 4. RMS Noise vs. Input Bandwidth of ADC
100
300
200
0
1k
10k
FREQUENCY – Hz
100k
1M
10M
–7–
The accuracy in sampling high frequency signals is also
constrained by the distortion and noise created by the
sample-and-hold. The level of distortion increases with
frequency and reduces the “effective number of bits” of the
conversion.
Measurements of Figures 6 and 7 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 @
Figure 7. Signal/(Noise and Distortion) vs. Frequency
10 BITS
12 BITS
14 BITS
Figure 6. Total Harmonic Distortion vs. Frequency
8 BITS
60
–70
–90
–95
90
80
70
50
40
30
20
10
–65
–75
–80
–85
Figure 5. Error Magnitude vs. Frequency
0
1k
0.01%
100
0.1%
1%
1k
APERTURE JITTER TYPICAL AT 20ps
IN
1k
= 5 V p-p and a sample frequency of
10k
FREQUENCY – Hz
FREQUENCY – Hz
10k
FREQUENCY – Hz
10k
100k
100k
100k
AD783
1M
1M
1M