Datasheet

Data Sheet ADuC7019/20/21/22/24/25/26/27/28/29
Rev. F | Page 49 of 104
Pseudo Differential Mode
In pseudo differential mode, Channel− is linked to the V
IN−
pin
of the ADuC7019/20/21/22/24/25/26/27/28/29. SW2 switches
between A (Channel−) and B (V
REF
). The V
IN−
pin must be
connected to ground or a low voltage. The input signal on V
IN+
can then vary from V
IN−
to V
REF
+ V
IN−
. Note that V
IN−
must be
chosen so that V
REF
+ V
IN−
does not exceed AV
DD
.
04955-019
CAPACITIVE
DAC
CAPACITIVE
DAC
CONTROL
LOGIC
COMPARATOR
SW3
SW1
A
A
B
B
SW2
C
S
C
S
V
REF
AIN0
AIN11
V
IN–
MUX
CHANNEL+
CHANNEL–
Figure 56. ADC in Pseudo Differential Mode
Single-Ended Mode
In single-ended mode, SW2 is always connected internally to
ground. The V
IN−
pin can be floating. The input signal range on
V
IN+
is 0 V to V
REF
.
04955-020
CAPACITIVE
DAC
CAPACITIVE
DAC
CONTROL
LOGIC
COMPARATOR
SW3
SW1
A
B
C
S
C
S
AIN0
AIN11
MUX
CHANNEL+
CHANNEL–
Figure 57. ADC in Single-Ended Mode
Analog Input Structure
Figure 58 shows the equivalent circuit of the analog input structure
of the ADC. The four diodes provide ESD protection for the analog
inputs. Care must be taken to ensure that the analog input
signals never exceed the supply rails by more than 300 mV;
exceeding 300 mV causes these diodes to become forward-
biased and start conducting into the substrate. These diodes can
conduct up to 10 mA without causing irreversible damage to
the part.
The C1 capacitors in Figure 58 are typically 4 pF and can be
primarily attributed to pin capacitance. The resistors are
lumped components made up of the on resistance of the
switches. The value of these resistors is typically about 100 Ω.
The C2 capacitors are the ADC’s sampling capacitors and
typically have a capacitance of 16 pF.
A
V
DD
C1
D
D
R1
C2
AV
DD
C1
D
D
R1
C2
04955-021
Figure 58. Equivalent Analog Input Circuit Conversion Phase: Switches Open,
Track Phase: Switches Closed
For ac applications, removing high frequency components from
the analog input signal is recommended by using an RC low-
pass filter on the relevant analog input pins. In applications
where harmonic distortion and signal-to-noise ratio are critical,
the analog input should be driven from a low impedance
source. Large source impedances significantly affect the ac
performance of the ADC. This can necessitate the use of an
input buffer amplifier. The choice of the op amp is a function of
the particular application. Figure 59 and Figure 60 give an
example of an ADC front end.
04955-061
ADuC7019/
ADuC702x
ADC0
10Ω
0.01µF
Figure 59. Buffering Single-Ended/Pseudo Differential Input
04955-062
ADuC7019/
ADuC702x
ADC0
V
REF
ADC1
Figure 60. Buffering Differential Inputs
When no amplifier is used to drive the analog input, the source
impedance should be limited to values lower than 1 kΩ. The
maximum source impedance depends on the amount of total
harmonic distortion (THD) that can be tolerated. The THD
increases as the source impedance increases and the performance
degrades.
DRIVING THE ANALOG INPUTS
Internal or external references can be used for the ADC. In
the differential mode of operation, there are restrictions on the
common-mode input signal (V
CM
), which is dependent upon
the reference value and supply voltage used to ensure that the
signal remains within the supply rails. Table 28 gives some
calculated V
CM
minimum and V
CM
maximum values.