AD7827_15 Datasheet, PDF (7/12 Pages) Analog Devices – 3 V/5 V, 1 MSPS, 8-Bit, Serial Interface Sampling ADC

AD7827
ADC TRANSFER FUNCTION
The output coding of the AD7827 is straight binary. The
designed code transitions occur at successive integer LSB
values (i.e., 1 LSB, 2 LSBs, etc.). The LSB size is = V_REF/256
(V_DD = 5 V) or the LSB size = (0.8 V_REF)/256 (V_DD = 3 V). The
ideal transfer characteristic for the AD7827 is shown in Figure 6
below.
11111111
111....110
(V_DD = 5V)
1LSB = V_REF/256
111....000
10000000
000....111
(V_DD = 3V)
1LSB = 0.8V_REF/256
000....010
000....001
00000000
1LSB (V_DD = 5V) V_REF/2
(V_DD = 3V) 0.8V_REF/2
(V_DD = 5V) V_REF/2 – 1.25V
(V_DD = 3V) 0.8V_REF/2 – 1V
V_REF/2+1.25V – 1LSB
0.8V_REF/2+1V – 1LSB
Figure 6. Transfer Characteristic
ANALOG INPUT
The AD7827 has a single input channel with an input range of
0 V to 2.5 V or 0 V to 2.0 V, depending on the supply voltage
(V_DD). This input range is automatically set up by an on-chip
“V_DD detector” circuit. 5 V operation of the ADC is detected
when V_DD exceeds 4.1 V and 3 V operation is detected when
V_DD falls below 3.8 V. This circuit also possesses a degree of
glitch rejection; for example, a glitch from 5.5 V to 2.7 V up to
60 ns wide will not trip the V_DD detector.
Note: Although there is a V_REF pin from which a voltage refer-
ence of 2.5 V may be sourced, or to which an external reference
may be applied, this does not provide an option of varying the value
of the voltage reference. As stated in the specifications for the
AD7827, the input voltage range at this pin is 2.5 V ± 2%.
Analog Input Structure
Figure 7 shows an equivalent circuit of the analog input struc-
ture of the AD7827. The two diodes, D1 and D2, provide ESD
protection for the analog inputs. Care must be taken to ensure
that the analog input signal never exceeds the supply rails by
more than 200 mV. This will cause these diodes to become
forward biased and start conducting current into the substrate.
The maximum current these diodes can conduct without caus-
ing irreversible damage to the part is 20 mA. The capacitor C2
in Figure 7 is typically about 4 pF and can mostly be attributed
to pin capacitance. The resistor R1 is a lumped component
made up of the on resistance of several components including
that of the multiplexer and the track-and-hold. This resistor is
typically about 310 Ω. The capacitor C1 is the track-and-hold
capacitor and has a capacitance of 0.5 pF. Switch 1 is the track-
and-hold switch, while Switch 2 is that of the sampling capaci-
tor as shown in Figures 2 and 3.
When in track phase, Switch 1 is closed and Switch 2 is in
Position A. When in hold mode, Switch 1 opens while Switch 2
remains in Position A. The track-and-hold remains in hold
mode for 120 ns—see Circuit Description, after which it returns
to track mode and the ADC enters its conversion phase. At this
point Switch 1 opens and Switch 2 moves to Position B. At the
end of the conversion Switch 2 moves back to Position A.
V_IN
C2
4pF
V_DD
D1
R1
310⍀
C1
0.5pF _A SW2
SW1
D2
B
Figure 7. Equivalent Analog Input Circuit
The on-chip track-and-hold can accommodate input frequen-
cies to 10 MHz, making the AD7827 ideal for subsampling
applications. When the AD7827 is converting a 10 MHz input
signal at a sampling rate of 1 MSPS, the effective number of
bits typically remains above seven corresponding to a signal-to-
noise ratio of 42 dBs as shown in Figure 8.
50
F_SAMPLE = 1MHz
48
46
44
42
40
38
0.2
1
3
4
5
6
INPUT FREQUENCY – MHz
8
10
Figure 8. SNR vs. Input Frequency On the AD7827
REV. 0
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