AD7608 Analog Devices, AD7608 Datasheet - Page 20

• Manufacturer Part Number: AD7608
• Description: 8-Channel DAS with 18-Bit, Bipolar, Simultaneous Sampling ADC
• Manufacturer: Analog Devices

Specifications of AD7608

• Resolution (bits): 18bit
• # Chan: 8
• Sample Rate: 200kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: Diff-Bip
• Ain Range: Bip 10V,Bip 5.0V
• Adc Architecture: SAR
• Pkg Type: QFP

AD7608
Analog Input Antialiasing Filter
An analog antialiasing filter (a second-order Butterworth) is also
provided on the AD7608. Figure 34 and Figure 35 show the
frequency and phase response, respectively, of the analog
antialiasing filter. In the ±5 V range, the −3 dB frequency is
typically 15 kHz. In the ±10 V range, the −3 dB frequency is
typically 23 kHz.
Track-and-Hold Amplifiers
The track-and-hold amplifiers on the AD7608 allow the ADC
to 18-bit resolution. The track-and-hold amplifiers sample
their respective inputs simultaneously on the rising edge of
CONVST x. The aperture time for track-and-hold (that is, the
delay time between the external CONVST x signal and the
to accurately acquire an input sine wave of full-scale amplitude
–10
–15
–20
–25
–30
–35
–40
18
16
14
12
10
–2
–5
8
6
4
2
0
5
0
100
100
Figure 34. Analog Antialiasing Filter Frequency Response
Figure 35. Analog Antialiasing Filter Phase Response
AV
f
T
SAMPLE
±10V RANGE
±5V RANGE
AV
f
T
A
±10V RANGE
SAMPLE
±5V RANGE
A
CC
= 25°C
CC
= 25°C
, V
, V
DRIVE
= 200kSPS
DRIVE
= 200kSPS
–40
+25
+85
–40
+25
+85
= 5V
= 5V
0.1dB
10,303Hz 24,365Hz
9619Hz
9326Hz
0.1dB
5225Hz
5225Hz
4932Hz
INPUT FREQUENCY (Hz)
INPUT FREQUENCY (Hz)
1k
1k
3dB
23,389Hz
22,607Hz
3dB
16,162Hz
15,478Hz
14,990Hz
±5V RANGE
10k
10k
±10V RANGE
100k
100k
Rev. A | Page 20 of 32
track-and-hold actually going into hold) is well matched, by design,
across all eight track-and-holds on one device and from device
to device. This matching allows more than one AD7608 device
to be sampled simultaneously in a system.
The end of the conversion process across all eight channels is
indicated by the falling edge of BUSY; and it is at this point that the
track-and-holds return to track mode, and the acquisition time
for the next set of conversions begins.
The conversion clock for the part is internally generated, and
the conversion time for all channels is 4 µs on the AD7608. The
BUSY signal returns low after all eight conversions to indicate the
end of the conversion process. On the falling edge of BUSY, the
track-and-hold amplifiers return to track mode. New data can
be read from the output register via the parallel, parallel byte, or
serial interface after BUSY goes low; or, alternatively, data from
the previous conversion can be read while BUSY is high. Reading
data from the AD7608 while a conversion is in progress has little
effect on performance and allows a faster throughput to be
achieved. In parallel mode at V
by ~1.5 dB when reading during a conversion.
ADC TRANSFER FUNCTION
The output coding of the AD7608 is twos complement. The
designed code transitions occur midway between successive
integer LSB values, that is, 1/2 LSB, 3/2 LSB. The LSB size is
FSR/262,144 for the AD7608. The ideal transfer characteristic
for the AD7608 is shown in Figure 36.
The LSB size is dependent on the analog input range selected.
±10V RANGE +10V
±5V RANGE
011...111
011...110
000...001
000...000
111...111
100...010
100...001
100...000
Figure 36. AD7608 Transfer Characteristic
–FS + 1/2LSB
+FS
+5V
±10V CODE =
±5V CODE =
MIDSCALE
0V
0V
0V – 1LSB +FS – 3/2LSB
ANALOG INPUT
10V
DRIVE
VIN
VIN
5V
× 131,072 ×
× 131,072 ×
–FS
–10V
–5V
> 3.3 V, the SNR is reduced
LSB =
REF
2.5V
REF
2.5V
76.29µV
38.15µV
LSB
+FS – (–FS)
Data Sheet
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