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AD7741/AD7742
Table II. AD7741/AD7742 Input Range Selection
UNI/ BIP
N/A
0
0
1
1
GAIN
N/A
0
1
0
1
Gain, G
X1
X1
X2
X1
X2
V IN (Min)
f OUT = 0.05 f CLKIN
0
–V REF
–V REF /2
0
0
V IN (Max)
f OUT = 0.45 f CLKIN
+V REF
+V REF
+V REF /2
+V REF
+V REF /2
Part
AD7741
AD7742
AD7742
AD7742
AD7742
As can be seen from Table II, the AD7741 has one input range
configuration whereas the AD7742 has unipolar/bipolar as
well as gain options depending on the status of the GAIN
and UNI/ BIP pins.
The transfer function for the AD7741 is shown in Figure 3.
Figure 4 shows the AD7742 transfer function for unipolar input
range configuration while the AD7742 transfer function for
bipolar input range configuration is shown in Figure 5.
OUTPUT
FREQUENCY
f OUT
OUTPUT
FREQUENCY
f OUT
f OUT MAX
(0.45 f CLKIN )
f OUT MIN
(0.05 f CLKIN )
f OUT MAX
–
V REF
GAIN
+
V REF
GAIN
DIFFERENTIAL
INPUT VOLTAGE
(0.45 f CLKIN )
Figure 5. AD7742 Transfer Characteristic for Bipolar
Differential Input Range: –V REF /Gain to +V REF /Gain; the
common-mode range must be between +0.5 V and
V DD – 1.75 V. UNI/ BIP pin tied to GND.
f OUT MIN
(0.05 f CLKIN )
0
REFIN
INPUT
VOLTAGE V IN
VFC Modulator
The analog input signal to the AD7741/AD7742 is continu-
ously sampled by a switched capacitor modulator whose sam-
pling rate is set by a master clock input that may be supplied
Figure 3. AD7741 Transfer Characteristic for Input Range
from 0 to V REF
OUTPUT
FREQUENCY
f OUT
f OUT MAX
(0.45 f CLKIN )
f OUT MIN
(0.05 f CLKIN )
externally or by a crystal-controlled on-chip clock oscillator.
However, the input signal is buffered on-chip before being ap-
plied to the sampling capacitor of the modulator. This isolates
the sampling capacitor charging currents from the analog input
pins.
This system is a negative feedback loop that tries to keep the net
charge on the integrator capacitor at zero, by balancing charge
injected by the input voltage with charge injected by the V REF .
The output of the comparator provides the digital input for the
1-bit DAC, so that the system functions as a negative feedback
loop that tries to minimize the difference signal (see Figure 6).
CLK
INTEGRATOR
COMPARATOR
0
+
V REF
GAIN
DIFFERENTIAL
INPUT VOLTAGE
INPUT
+
–
+
1-BIT
STREAM
Figure 4. AD7742 Transfer Characteristic for Unipolar
Differential Input Range: 0 V to V REF /Gain; the input
common-mode range must be between +0.5 V and
V DD – 1.75 V. UNI/ BIP pin tied to V DD .
–8 –
–
+V REF
–V REF
Figure 6. AD7741/AD7742 Modulator Loop
REV. 0
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