Datasheet
ZADCS146 / ZADCS147
Figure 4: Block diagram of input multiplexer
Figure 5: Input voltage range in unipolar mode
VIN+
Shown configuration
A2 … A0 = 0x000
1.5*VREF
0xFFF
CH0
CH1
CH2
CH3
CH4
VREF
Code Range
CH5
0.5*VREF
IN+
Converter
CH6
CH7
0x000
IN-
0V
VDD-VREF VIN-
Figure 6: Input voltage range for fully differen-
tial signals in bipolar mode
VCM
VREF
¾ VREF
VCM Range
COM
¼ VREF
0V
SGL/DIF = HIGH
See Table 3 & Table 4
for Coding Schemes
-VREF/2
0V
+VREF/2
VDIFF
voltage at IN– to obtain codes unequal to 0x000. The
entire 12 bit transfer characteristic is then covered by IN+
if IN+ ranges from IN– to (IN– +Vref). Any voltage on
IN+ > (IN– + Vref) results in code 0xFFF. Code 0xFFF is
not reached, if (IN– +Vref) > VDD + 0.2V because the
input voltage is clamped at VDD + 0.2V by ESD protec-
tion devices.
the peak to peak amplitude of the differential input signal
can be ± Vref/2.
The average input current on the analog inputs depends
on the conversion rate. The signal source must be capa-
ble of charging the internal sampling capacitors (typically
16pF on each input of the converter: IN+ and IN–) within
the acquisition time tACQ to the required accuracy. The
equivalent input circuit in sampling mode is shown in
Figure 7.
The voltage at IN– can range from -0.2V … ½ VREF with-
out limiting the Code Range, assuming the fore men-
tioned VDD condition is true. See also Figure 5 for input
voltage ranges in unipolar conversion mode.
The following equation provides a rough hand calculation
for a source impedance RS that is required to settle out a
DC input signal referenced to AGND with 12 bit accuracy
in a given acquisition time
In bipolar mode, IN+ can range from (IN– - Vref/2) to (IN–
+ Vref/2) keeping the converter out of code saturation.
For instance, if IN– is set to a constant DC voltage of
Vref/2, then IN+ can vary from 0V to Vref to cover the
entire code range. Lower or higher voltages of IN+ keep
the output code at the minimum or maximum code value.
CHOLD+
RSW
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
Figure 6 shows the input voltage ranges in bipolar mode
when IN– is set to a constant DC voltage.
IN+
CIN
4pF
16pF
3kΩ
As explained before, ZADCS146 / ZADCS147 can also
be used to convert fully differential input signals that
change around a common mode input voltage.
AGND
CHOLD-
VDC
RSW
The bipolar mode is best used for such purposes since it
allows the input signals to be positive or negative in rela-
tion to each other.
COM
Channel
Multiplexer
IN-
CIN
4pF
AGND
16pF
3kΩ
The common mode level of a differential input signal is
calculated VCM = (V(IN+)+ V(IN–)) / 2. To avoid code clip-
ping or over steering of the converter, the common mode
level can change from ¼ Vref … ¾ Vref. Within this range
Figure 7: Equivalent input circuit during sampling
Copyright © 2008, ZMD AG, Rev. 1.1
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Information furnished in this publication is preliminary and subject to changes without notice.
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