ADUC7025BCPZ32-RL7 AD [Analog Devices], ADUC7025BCPZ32-RL7 Datasheet - Page 41

• Manufacturer Part Number: ADUC7025BCPZ32-RL7
• Description: Precision Analog Microcontroller 12-bit Analog I/O, ARM7TDMI MCU
• Manufacturer: AD [Analog Devices]

AIN11
Pseudo Differential Mode
In pseudo differential mode, Channel− is linked to the V
of the ADuC7019/7020/7021/7022/7024/7025/7026/7027. SW2
switches between A (Channel−) and B (V
connected to ground or a low voltage. The input signal on V
can then vary from V
chosen so that V
AIN11
Single-Ended Mode
In single-ended mode, SW2 is always connected internally to
ground. The V
V
AIN11
Analog Input Structure
Figure 46 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; this
would cause 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.
AIN0
AIN0
AIN0
VIN–
IN+
is 0 V to V
MUX
MUX
MUX
CHANNEL+
CHANNEL–
CHANNEL+
CHANNEL–
CHANNEL+
Figure 44. ADC in Pseudo Differential Mode
IN−
REF
REF
Figure 45. ADC in Single-Ended Mode
pin can be floating. The input signal range on
CHANNEL–
.
Figure 43. ADC Conversion Phase
+ V
B
A
A
B
B
A
A
B
B
A
IN−
V
V
REF
REF
SW1
SW2
SW1
SW2
SW1
IN−
to V
do not exceed AV
C
C
C
C
C
C
REF
S
S
S
S
S
S
+ V
IN−
SW3
SW3
SW3
. Note that V
COMPARATOR
COMPARATOR
COMPARATOR
REF
). V
DD
.
IN−
pin must be
IN−
CAPACITIVE
CAPACITIVE
CAPACITIVE
CAPACITIVE
CAPACITIVE
CAPACITIVE
CONTROL
CONTROL
CONTROL
must be
DAC
DAC
DAC
DAC
DAC
DAC
LOGIC
LOGIC
LOGIC
IN−
pin
IN+
Rev. A | Page 41 of 92
The capacitors, C1, in Figure 46 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 capacitors, C2, are the ADC’s sampling capacitors and
typically have a capacitance of 16 pF.
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 47 and Figure 48 give an
example of ADC front end.
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.
Figure 46. Equivalent Analog Input Circuit Conversion Phase:
Figure 47. Buffering Single-Ended/Pseudo Differential Input
ADuC7019/20/21/22/24/25/26/27
Switches Open; Track Phase: Switches Closed
V
REF
Figure 48. Buffering Differential Inputs
C1
C1
AV
AV
10Ω
DD
DD
0.01μF
D
D
D
D
ADuC702x
ADC0
ADC1
R1 C2
R1 C2
ADuC702x
ADC0