AD7705/AD7706
Pin No.
10
Mnemonic
REF IN(–)
Function
Reference Input. Negative input of the differential reference input to the AD7705/AD7706.
The REF IN(–) can lie anywhere between V
DD
and GND provided REF IN(+) is greater
than REF IN(–).
AD7705: Negative input of the differential analog Input Channel 2. AD7706: Analog Input
Channel 3.
Logic Output. A logic low on this output indicates that a new output word is available from
the AD7705/AD7706 data register. The
DRDY
pin will return high upon completion of a
read operation of a full output word. If no data read has taken place between output updates,
the
DRDY
line will return high for 500
×
t
CLK IN
cycles prior to the next output update.
While
DRDY
is high, a read operation should neither be attempted nor in progress to avoid
reading from the data register as it is being updated. The
DRDY
line will return low again
when the update has taken place.
DRDY
is also used to indicate when the AD7705/AD7706
has completed its on-chip calibration sequence.
Serial Data Output with serial data being read from the output shift register on the part. This
output shift register can contain information from the setup register, communications regis-
ter, clock register or data register, depending on the register selection bits of the Communica-
tions Register.
Serial Data Input with serial data being written to the input shift register on the part. Data
from this input shift register is transferred to the setup register, clock register or communica-
tions register, depending, on the register selection bits of the Communications Register.
Supply Voltage, +2.7 V to +5.25 V operation.
Ground reference point for the AD7705/AD7706’s internal circuitry.
11
12
AIN2(–)[AIN3]
DRDY
13
DOUT
14
DIN
15
16
V
DD
GND
OUTPUT NOISE (5 V OPERATION)
Table I shows the AD7705/AD7706 output rms noise for the selectable notch and –3 dB frequencies for the part, as selected by FS0
and FS1 of the Clock Register. The numbers given are for the bipolar input ranges with a V
REF
of +2.5 V and V
DD
= 5 V. These
numbers are typical and are generated at an analog input voltage of 0 V with the part used in either buffered or unbuffered mode. Table II
meanwhile shows the output
peak-to-peak
noise for the selectable notch and –3 dB frequencies for the part.
It is important to note that
these numbers represent the resolution for which there will be no code flicker. They are not calculated based on rms noise but on peak-to-peak
noise.
The numbers given are for bipolar input ranges with a V
REF
of +2.5 V and for either buffered or unbuffered mode. These num-
bers are typical and are rounded to the nearest LSB. The numbers apply for the CLK DIV bit of the Clock Register set to 0.
Table I. Output RMS Noise vs. Gain and Output Update Rate @ 5 V
Filter First
Notch and O/P –3 dB
Data Rate
Frequency
MCLK IN = 2.4576 MHz
50 Hz
13.1 Hz
60 Hz
15.72 Hz
250 Hz
65.5 Hz
500 Hz
131 Hz
MCLK IN = 1 MHz
20 Hz
5.24 Hz
25 Hz
6.55 Hz
100 Hz
26.2 Hz
200 Hz
52.4 Hz
Gain of
1
4.1
5.1
110
550
4.1
5.1
110
550
Gain of
2
2.1
2.5
49
285
2.1
2.5
49
285
Typical Output RMS Noise in V
Gain of
Gain of
Gain of
4
8
16
1.2
1.4
31
145
1.2
1.4
31
145
0.75
0.8
17
70
0.75
0.8
17
70
0.7
0.75
8
41
0.7
0.75
8
41
Gain of
32
0.66
0.7
3.6
22
0.66
0.7
3.6
22
Gain of
64
0.63
0.67
2.3
9.1
0.63
0.67
2.3
9.1
Gain of
128
0.6
0.62
1.7
4.7
0.6
0.62
1.7
4.7
REV. A
–7–
AD [ ANALOG DEVICES ]
