AD7821
PIN FUNCTION DESCRIPTION
OPERATING SEQUENCE
Pin
1
2
3–5
6
7
Mnemonic Description
V
IN
DB0
DB1–DB3
WR/RDY
MODE
Analog Input: Range V
REF
(–)
≤
V
IN
≤
V
REF
(+)
Three-State Data Output (LSB).
Three-State Data Outputs.
WRITE control input/READY status
output. See Digital Interface section.
Mode Selection Input. It determines
whether the device operates in the
WR-RD or RD mode. This input is in-
ternally pulled low through a 50
µA
current source. See Digital Interface
section.
READ Input.
RD
must be low to access
data from the part. See Digital Interface
section.
INTERRUPT Output.
INT
going low
indicates that the conversion is complete.
INT
returns high on the rising edge of
CS
or
RD.
See Digital Interface section.
Ground.
Lower limit of reference span.
Range: V
SS
≤
V
REF
(–)
≤
V
REF
(+).
Upper limit of reference span.
Range: V
REF
(–) < V
REF
(+)
≤
V
DD
.
Chip Select Input. The device is selected
when this input is low.
Three-State Data Outputs.
Three-State Data Output (MSB).
Overflow Output. If the analog input is
higher than (V
REF
(+) – 1/2 LSB),
OFL
will be low at the end of conversion. It is
a non-three-state output which can be
used to cascade 2 or more devices to
increase resolution.
Negative supply voltage.
V
SS
= 0 V; Unipolar Operation.
V
SS
= –5 V; Bipolar Operation.
Positive supply voltage, +5 V.
The AD7821 has two operating modes. The RD mode allows a
conversion to be started and data to be read with a single, ex-
tended, READ operation (i.e.,
CS
and
RD
are taken low). The
conversion process is timed out by internal one-shots. The WR-
RD mode uses
WR
to start a conversion and
RD
to read the
data and allows the conversion timing to be externally con-
trolled. The operating sequence for the WR-RD mode is shown
in Figure 3.
8
RD
Figure 3. Operating Sequence (WR-RD Mode)
9
INT
10
11
12
13
14–16
17
18
GND
V
REF
(–)
V
REF
(+)
CS
DB4–DB6
DB7
OFL
A conversion is initiated and the analog input signal (V
IN
)
sampled on the falling edge of
WR
(falling edge of
RD,
RD
mode). A setup time (t
P
, delay time between conversions) of
350 ns is required prior to this falling edge. See Digital Interface
section for more details. When
WR
is low, the internal MS
(most significant) ADC compares the sampled analog input with
the reference ladder to provide the 4 MS data bits. A minimum
of 250 ns is required for this comparison. On the rising edge of
WR,
the MS data result is latched internally and the LS (least
significant) conversion begins, to yield the 4 LS data bits.
INT
goes low typically 380 ns after the rising edge of
WR.
This indi-
cates the LS conversion is complete and that both the LS and
MS data results are latched into the output buffer.
RD
going
low then enables the output data. If a faster conversion time is
required, the
RD
line can be brought low 250 ns after
WR
goes
high. This latches both the LS and MS data bits and outputs the
conversion result on DB0–DB7.
REFERENCE AND INPUT
19
V
SS
The V
REF
(–) and V
REF
(+) reference inputs on the AD7821 are
fully differential and define the zero and full-scale input range of
the ADC. The transfer characteristic of the part is defined by
the integer value of the following expression:
V
IN
−
V
REF
(−)
Data
(LSBs )
=
256
+
0.5
V
REF
(+)
−
V
REF
(−)
20
V
DD
CIRCUIT INFORMATION
BASIC DESCRIPTION
The AD7821 uses a half flash conversion technique (see Func-
tional Block Diagram), whereby two 4-bit flash ADCs are used
to achieve an 8-bit result. Each 4-bit flash ADC contains 15
comparators, which compare an unknown input voltage to the
reference ladder, to achieve a 4-bit result. The MS (most signifi-
cant) flash ADC converts an unknown analog input voltage
(V
IN
) to provide the 4 MS data bits. An internal DAC, driven by
the 4 MS data bits, then recreates an analog approximation of
the input voltage. The DAC output voltage is subtracted from
the analog input, and the difference is converted by the LS
(least significant) ADC to provide the 4 LS data bits. The MS
flash ADC also has one additional comparator to detect over-
range on the analog input.
As a result, the analog input (V
IN
) Of the device can easily be set
up to provide both unipolar and bipolar operation. The data
output code for unipolar and bipolar operation is Natural Binary
and Offset Binary, respectively.
The span of the analog input voltage can easily be varied.
By reducing the reference span, V
REF
(+) – V
REF
(–), to less than
5 V the sensitivity of the converter can be increased (i.e., if
V
REF
= 2 V then 1 LSB = 7.8 mV). The reference flexibility also
allows the input span for unipolar operation to be offset from
zero (V
REF
(–) > GND). Additionally, the input/reference ar-
rangement facilitates ratiometric operation.
Figures 4 and 5 show some configurations which are possible.
For minimum noise a 47
µF
capacitor in parallel with a 0.1
µF
capacitor should be connected between the reference inputs and
GND.
–6–
REV. A
AD [ ANALOG DEVICES ]
