EVAL-AD7741-42EBZ Datasheet, PDF (8/12 Pages) Analog Devices – Single and Multichannel, Synchronous Voltage-to-Frequency Converters

AD7741/AD7742
UNI/BIP
N/A
0
0
1
1
GAIN
N/A
0
1
0
1
Table II. AD7741/AD7742 Input Range Selection
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
f_OUTMAX
(0.45 f_CLKIN)
f_OUTMIN
(0.05 f_CLKIN)
0
REFIN
INPUT
VOLTAGE V_IN
Figure 3. AD7741 Transfer Characteristic for Input Range
from 0 to V_REF
OUTPUT
FREQUENCY
f_OUT
f_OUTMAX
(0.45 f_CLKIN)
f_OUTMIN
(0.05 f_CLKIN)
0
+ ^VREF
GAIN
DIFFERENTIAL
INPUT VOLTAGE
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.
OUTPUT
FREQUENCY
f_OUT
f_OUTMAX
(0.45 f_CLKIN)
f_OUTMIN
(0.05 f_CLKIN)
– ^VREF
GAIN
+ ^VREF
GAIN
DIFFERENTIAL
INPUT VOLTAGE
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.
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
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).
INPUT
+
⌺
–
INTEGRATOR
CLK
COMPARATOR
+
–
+V_REF
1-BIT
STREAM
–V_REF
Figure 6. AD7741/AD7742 Modulator Loop
–8–
REV. 0