AD7898 Analog Devices, AD7898 Datasheet - Page 9

• 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

Figure 2 shows the analog input section for the AD7898-10
and AD7898-3. The analog input range of the AD7898-10 is
± 10 V into an input resistance of typically 30 kΩ. The analog
input range of the AD7898-3 is ± 2.5 V into an input resistance
of typically 6 kΩ. This input is benign, with no dynamic charg-
ing currents, as the resistor stage is followed by a high input
impedance stage of the track/hold amplifier. For the AD7898-10,
R1 = 30 kΩ, R2 = 7.5 kΩ and R3 = 10 kΩ. For the AD7898-3,
R1 = R2 = 6.5 kΩ and R3 is open circuit.
For the AD7898-10 and AD7898-3, the designed code transi-
tions occur midway between successive LSB values (i.e., 1/2 LSB,
3/2 LSBs, 5/2 LSBs . . .). Output coding is twos complement
binary with 1 LSB = FS/4096. For the AD7898-10 1 LSB = 20/
4096 = 4.88 mV. For the AD7898-3 1 LSB = 5/4096 = 1.22 mV.
The ideal input/output coding for the AD7898-10 and AD7898-3
is shown in Table I.
Table I. Ideal Input/Output Code Table for the AD7898-10/
AD7898-3 Digital Output
Analog Input
+FSR/2 – 3/2 LSB
+FSR/2 – 5/2 LSBs
+FSR/2 – 7/2 LSBs
AGND + 3/2 LSB
AGND + 1/2 LSB
AGND – 1/2 LSB
AGND – 3/2 LSB
–FSR/2 + 5/2 LSBs
–FSR/2 + 3/2 LSBs
–FSR/2 + 1/2 LSB
NOTES
1
2
REV. A
FSR is full-scale range = 20 V (AD7898-10) and = 5 V (AD7898-3) with
1 LSB = FSR/4096 = 4.883 mV (AD7898-10) and 1.22 mV (AD7898-3) with
REF IN = 2.5 V.
REF IN = 2.5 V.
Figure 3. THD vs. Source Impedance for Various Analog
Input Frequencies
–60
–65
–70
–75
–80
–85
–90
10
f
l
IN
= 10k
2
SOURCE IMPEDANCE –
100
f
IN
= 50k
f
IN
Code Transition
011 . . . 110 to 011 . . . 111
011 . . . 101 to 011 . . . 110
011 . . . 100 to 011 . . . 101
000 . . . 001 to 000 . . . 010
000 . . . 000 to 000 . . . 001
111 . . . 111 to 000 . . . 000
111 . . . 110 to 111 . . . 111
100 . . . 010 to 100 . . . 011
100 . . . 001 to 100 . . . 010
100 . . . 000 to 100 . . . 001
= 25k
1k
f
IN
= 110k
10k
–9–
Figure 3 shows a graph of THD versus source impedance for
different analog input frequencies when using a supply voltage
of 5 V, V
Source impedance has a minimal effect on THD because of the
resistive ladder structure of the input section of the ADC. Figure 4
shows a graph of THD versus Analog input frequency for vari-
ous supply voltages while sampling at 220 kSPS.
Acquisition Time
The track-and-hold amplifier enters its tracking mode on the
falling 14th SCLK edge after the CS falling edge for Mode 1
operation. The time required for the track-and-hold amplifier to
acquire an input signal will depend on how quickly the 9.1 pF
sampling capacitance is charged. With zero source impedance
on the analog input, two SCLK cycles plus t
be sufficient to acquire the signal to the 12-bit level. With an
SCLK frequency of 3.7 MHz, the acquisition time would be
2 × (270 ns) + t
The acquisition time required is calculated using the following
formula:
where R is the resistance seen by the track-and-hold amplifier
looking back on the input e.g., for AD7898-10 R = 3.75 kΩ and
for AD7898-3 R = 3.25 kΩ. The sampling capacitor has a value
of 9.1 pF. Theoretical acquisition times would be 340 ns for
AD7898-10, and 295 ns for AD7898-3. These theoretical values
do not include t
typical values would be 520 ns for the AD7898-10 and 450 ns
for the AD7898-3.
Figure 4. THD vs. Analog Input Frequency for Various
Supply Voltages
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
10
DRIVE
t
ACQ
V
DD
= V
= 10 × (RC)
QUIET.
QUIET
of 5 V, and sampling at a rate of 220 kSPS.
DRIVE
= 4.75V
or track propagation delays in the part,
INPUT FREQUENCY – kHz
100
V
V
DD
DD
= 5.0V, V
= V
DRIVE
DRIVE
= 5.25V
QUIET
AD7898
= 3.0V
will always
1000