AD7898 Analog Devices, AD7898 Datasheet - Page 8

• Manufacturer Part Number: AD7898
• Description: 5V, 12-Bit, Serial 220 kSPS ADC in a 8-Lead Package
• Manufacturer: Analog Devices

Specifications of AD7898

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 220kSPS
• Interface: Ser
• Analog Input Type: SE-Bip
• Ain Range: Bip (Vref),Bip (Vref) x 4,Bip 10V,Bip 2.5V
• Adc Architecture: SAR
• Pkg Type: SOIC

AD7898
TPC 5 shows a graph of Signal to (Noise + Distortion)
ratio versus Input Frequency for various supply voltages
while sampling at 220 kSPS. The on-chip track-and-hold
can accommodate frequencies up to 4.7 MHz for AD7898-3,
and up to 3.6 MHz for AD7898-10, making the AD7898 ideal
for subsampling applications.
Noise
In an A/D converter, noise exhibits itself as a code uncertainty
in dc applications, and as the noise floor (in an FFT, for
example) in ac applications. In a sampling A/D converter like
the AD7898, all information about the analog input appears in
the baseband, from dc to half the sampling frequency. The input
bandwidth of the track/hold exceeds the Nyquist bandwidth
and, therefore, an antialiasing filter should be used to remove
unwanted signals above f
where such signals exist.
TPC 6 shows a histogram plot for 8192 conversions of a dc
input using the AD7898. The analog input was set at the center
of a code transition. It can be seen that almost all the codes
appear in one output bin, indicating very good noise perfor-
mance from the ADC.
TPC 6. Histogram of 8192 Conversions of a DC Input
TPC 5. SINAD vs. Input Frequency at 220 kSPS
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
–40
–45
–50
–55
–60
–65
–70
–75
500
0
10
2044
2045
2046
INPUT FREQUENCY – kHz
S
V
/2 in the input signal in applications
DD
= V
2047
DRIVE
100
= 4.75V
V
2048
DD
V
DD
= 5.0V, V
= V
DRIVE
2049
DRIVE
= 5.25V
2050
= 3.0V
1000
–8–
CONVERTER DETAILS
The AD7898 is a fast, 12-bit single supply A/D converter. It
provides the user with signal scaling, track/hold, A/D converter,
and serial interface logic functions on a single chip. The A/D
converter section of the AD7898 consists of a conventional
successive-approximation converter based around an R-2R
ladder structure. The signal scaling on the AD7898-10 and
AD7898-3 allows the part to handle ± 10 V and ± 2.5 V input
signals, respectively, while operating from a single 5 V supply.
The part requires an external 2.5 V reference. The reference
input to the part is buffered on-chip. The AD7898 has two
operating modes, an internal clocking mode using an on-chip
oscillator and an external clocking mode using the SCLK as
the master clock. The latter mode features a power-down
mechanism. These modes are discussed in more detail in the
Operating Modes section.
A major advantage of the AD7898 is that it provides all of the
above functions in an 8-lead SOIC package. This offers the user
considerable spacing saving advantages over alternative solutions.
The AD7898 consumes only 22.5 mW maximum, making it
ideal for battery-powered applications.
In Mode 0 operation, conversion is initiated on the AD7898 by
pulsing the CONVST input. On the falling edge of CONVST,
the on-chip track/hold goes from track to hold mode, and the
conversion sequence is started. The conversion clock for the
part is generated internally using a laser-trimmed clock oscilla-
tor circuit. Conversion time for the AD7898 is 3.3 µs, and the
quiet time is 0.1 µs. To obtain optimum performance from the
part in Mode 0, the read operation should not occur during the
conversion.
In Mode 1 operation, conversion is initiated on the AD7898 by
the falling edge of CS. Sixteen SCLK cycles are required to
complete the conversion and access the conversion result, after
which time CS may be brought high. The internal oscillator is
not used as the conversion clock in this mode as the SCLK is
used instead. The maximum SCLK frequency is 3.7 MHz in
Mode 1 providing a minimum conversion time of 4.33 µs. As in
Mode 0, another conversion should not be initiated during the
quiet time after the end of conversion.
Both of these modes of operation allow the part to operate
at throughput rates up to 220 kHz and achieve data sheet
specifications.
CIRCUIT DESCRIPTION
Analog Input Section
The AD7898 is offered as two part types: the AD7898-10,
which handles a ± 10 V input voltage range; the AD7898-3,
which handles input voltage range ± 2.5 V.
AGND
VREF
V
IN
Figure 2. Analog Input Structure
R1
R3
R2
AD7898-10/AD7898-3
TO ADC
REFERENCE
CIRCUITRY
TRACK/HOLD
TO INTERNAL
COMPARATOR
REV. A