Datasheet

AD7482
Rev. B | Page 16 of 20
Data must not be read from the AD7482 while a conversion is
taking place. For this reason, if operating the AD7482 at through-
put speeds greater than 2.5 MSPS, it is necessary to tie both the
CS
pin and
RD
pins on the AD7482 low and use a buffer on the
data lines. This situation may also arise in the case where a read
operation cannot be completed in the time after the end of one
conversion and the start of the quiet period before the next
conversion.
The maximum slew rate at the input of the ADC must be limited to
500 V/µs while
BUSY
is low to avoid corrupting the ongoing
conversion. In any multiplexed application where the channel is
switched during conversion, this is to happen as soon as possible
after the
Reading Data from the AD7482
BUSY
falling edge.
Data is read from the part via a 13-bit parallel data bus with the
standard
CS
signal and
RD
signal. The
CS
signal and
The data lines D0 to D12 leave their high impedance state when
both the
RD
signal are
internally gated to enable the conversion result onto the data bus.
CS
and
RD
are logic low. Therefore,
CS
may be perma-
nently tied logic low if required, and the
Figure 29
RD
signal may be used
to access the conversion result.
shows a timing specifica-
tion called t
QUIET
. This is the amount of time that must be left
after any data bus activity before the next conversion is initiated.
Writing to the AD7482
The AD7482 features a user accessible offset register. This allows
the bottom of the transfer function to be shifted by ±200 mV. This
feature is explained in more detail in the Offset/Overrange section.
To write to the offset register, a 13-bit word is written to the
AD7482 with the 10 LSBs containing the offset value in twos
complement format. The 3 MSBs must be set to 0. The offset
value must be within the range −327 to +327, corresponding to
an offset from −200 mV to +200 mV. The value written to the
offset register is stored and used until power is removed from
the device, or the device is reset. The value stored can be updated at
any time between conversions by another write to the device.
Table 9 shows examples of offset register values and their effective
offset voltage. Figure 30 shows a timing diagram for writing to
the AD7482.
Table 9. Offset Register Examples
Code
(Decimal)
D12 to D10
D9 to D0 (Twos
Complement)
Offset
(mV)
−327 000 1010111001 −200
−128 000 1110000000 −78.12
[phone] +39.06
+327 000 0101000111 +200
Driving the
CONVST
To achieve the specified performance from the AD7482, the
Pin
CONVST
pin must be driven from a low jitter source. Because
the falling edge on the
( )
( )
2
2
1
log10dB
jIN
JITTER
tf
SNR
××π
=
CONVST
pin determines the sampling
instant, any jitter that may exist on this edge appears as noise
when the analog input signal contains high frequency components.
The relationship between the analog input frequency (f
IN
), timing
jitter (t
j
), and resulting SNR is given by
For example, if the desired SNR due to jitter was 100 dB with a
maximum full-scale analog input frequency of 1.5 MHz, ignoring
all other noise sources, the result is an allowable jitter on the
CONVST
falling edge of 1.06 ps. For a 12-bit converter (ideal
SNR = 74 dB), the allowable jitter is greater than 1.06 ps, but
due consideration must be given to the design of the
Typical Connection
CONVST
circuitry to achieve 12-bit performance with large analog input
frequencies.
Figure 23 shows a typical connection diagram for the AD7482
operating in Parallel Mode 1. Conversion is initiated by a falling
edge on
CONVST
. When
CONVST
goes low, the
BUSY
signal
goes low, and at the end of conversion, the rising edge of
BUSY
is used to activate an interrupt service routine. The
CS
and
In
RD
lines are then activated to read the 12 data bits (13 bits if using
the overrange feature).
Figure 23, the V
DRIVE
pin is tied to DV
DD
, which results in
logic output levels being either 0 V or DV
DD
. The voltage applied to
V
DRIVE
controls the voltage value of the output logic signals. For
example, if DV
DD
is supplied by a 5 V supply and V
DRIVE
is supplied
by a 3 V supply, the logic output levels are either 0 V or 3 V. This
feature allows the AD7482 to interface to 3 V devices, while still
enabling the ADC to process signals at a 5 V supply.
MICROCONTROLLER/
MICROPROCESSOR
RESET
PARALLEL
INTERFACE
MODE1
MODE2
WRITE
CLIP
NAP
STBY
D0 TO D12
CS
CONVST
RD
BUSY
C
BIAS
REFSEL
REFIN
REFOUT
VIN
AD7482
ADM809
V
DRIVE
DV
DD
AV
DD
0.1µF
DIGITAL
SUPPLY
4.75V TO 5.25V
10µF
1nF
+
0.1µF
0.1µF
+
47µF
ANALOG
SUPPLY
4.75V TO 5.25V
0V TO 2.5V
1nF
0.47µF
0.47µF
AD780 2.5V
REFERENCE
02638-023
Figure 23. Typical Connection Diagram