ad7883br Analog Devices, Inc., ad7883br Datasheet - Page 5

• Manufacturer Part Number: ad7883br
• Description: Lc2mos 12-bit, 3.3 V Sampling Adc
• Manufacturer: Analog Devices, Inc.

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CIRCUIT INFORMATION
The AD7883 is a single supply 12-bit A/D converter. The part
requires no external components apart from a 2 MHz external
clock and power supply decoupling capacitors. It contains a
12-bit successive approximation ADC based on a fast-settling
voltage output DAC, a high speed comparator and SAR, as well
as the necessary control logic. The charge balancing comparator
used in the AD7883 provides the user with an inherent track-
and-hold function. The ADC is specified to work with sampling
rates up to 50 kHz.
CONVERTER DETAILS
The AD7883 conversion cycle is initiated on the rising edge of
the CONVST pulse, as shown in the timing diagram of Figure
1. The rising edge of the CONVST pulse places the track/hold
amplifier into “HOLD” mode. The conversion cycle then takes
between 26 and 28 clock periods. The maximum specified con-
version time is 15 s. During conversion the BUSY output will
remain low, and the output databus drivers will be three-stated.
When a conversion is completed, the BUSY output will go to a
high level, and the result of the conversion can be read by bring-
ing CS and RD low.
The track/hold amplifier acquires a 12-bit input signal in 5 s.
The overall throughput time for the AD7883 is equal to the con-
version time plus the track/hold acquisition time. For a 2 MHz
input clock the throughput time is 20 s.
REFERENCE INPUT
For specified performance, it is recommended that the reference
input be tied to V
reference down to 2.5 V though with reduced performance
specifications.
V
ANALOG INPUT
The AD7883 has two analog input pins, V
3 shows the input circuitry to the ADC sampling comparator.
The onboard attenuator network, made up of equal resistors, al-
lows for various input ranges.
REF
must not be allowed to go above V
V
V
INA
INB
Figure 3. AD7883 Input Circuit
DD
. The part, however, will operate with a
R
R
V
DAC
DD
–
by more than 100 mV.
+
INA
and V
INB
. Figure
–5–
The AD7883 accommodates two separate input ranges, 0 to
V
these ranges are shown in Figures 4 and 5.
With V
ranges are 0 V to 3.3 V and 3.3 V, as shown in Table II.
Analog Input
Range
0 V to +3.3 V
3.3 V
REF
V
IN
and V
V
Figure 4. 0 to V
V
IN
= 0 TO V
REF
REF
Figure 5.
= V
= V
REF
V
REF
REF
REF
DD
. The input configurations corresponding to
Table II. Analog Input Ranges
V
V
V
and using a nominal V
REF
DD
DD
V
V
V
AGND
V
INA
INB
REF
V
V
AGND
V
REF
INA
INB
REF
REF
R
R
Bipolar Input Configuration
Unipolar Input Configuration
R
Input Connections
V
V
V
R
INA
IN
IN
12-BIT DAC
0 TO V
0 TO V
12-BIT DAC
DD
V
V
V
INB
IN
REF
REF
of +3.3 V, the input
REF
SAMPLING
COMPARATOR
+
–
AD7883
SAMPLING
COMPARATOR
+
–
Connection
Diagram
Figure 4
Figure 5