ADUC7122 Analog Devices, ADUC7122 Datasheet - Page 34

• Manufacturer Part Number: ADUC7122
• Description: Precision Analog Microcontroller, 12-Bit Analog I/O, ARM7TDMI® MCU
• Manufacturer: Analog Devices

Specifications of ADUC7122

• Mcu Core: ARM7 TDMI
• Mcu Speed (mips): 40
• Sram (bytes): 8192Bytes
• Gpio Pins: 32
• Adc # Channels: 13

ADuC7122
For ac applications, removing high frequency components from
the analog input signal is recommended through the use of 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 and can necessitate the use of an input buffer amplifier.
The choice of the op amp is a function of the particular application.
Figure 23 and Figure 24 give an example of an 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 22. Equivalent Analog Input Circuit Conversion Phase:
Figure 23. Buffering Single-Ended/Pseudo Differential Input
Switches Open, Track Phase: Switches Closed
V
REF
Figure 24. Buffering Differential Inputs
C1
C1
AV
AV
DD
DD
10Ω
D
D
D
D
0.01µF
R1 C2
R1 C2
ADuC7122
ADC0
ADC1
ADuC7122
ADC0
Rev. 0 | Page 34 of 96
DRIVING THE ANALOG INPUTS
Internal or external reference can be used for the ADC. In
differential mode of operation, there are restrictions on the
common-mode input signal (V
reference value and supply voltage used to ensure that the signal
remains within the supply rails.
Table 35 gives some calculated V
maximum values under various AV
Table 35. V
AV
3.3 V
3.0 V
BAND GAP REFERENCE
The ADuC7122 provides an on-chip band gap reference of
2.5 V that can be used for the ADC and for the DAC. This
2.5 V reference is generated from a 1.2 V reference.
This internal reference also appears on the V
V
capacitor must be connected between V
a 470 nF capacitor between V
stability and fast response during ADC conversions.
The band gap reference also connects through buffers to the
BUF_VREF1 and the BUF_VREF2 pins, which can be used as a
reference for other circuits in the system. A minimum of 0.1 μF
capacitor should be connected to these pins to damp noise.
The band gap reference interface consists of an 8-bit REFCON
MMR, described in Table 36. It is recommended to enable
REFCON Bit 0 and Bit 1 when performing an ADC or DAC
conversion that uses the internal reference.
An external reference can be used for an ADC conversion.
To perform an ADC conversion with an external 2.5 V refer-
ence, clear REFCON[1] and apply the external reference to
the VREF_2.5 pin. To apply an external 1.2 V reference, clear
REFCON[0] and apply the external reference to the V
Note that when applying an external reference to the V
pin, this internally influences the 2.5 V reference as the 2.5 V
reference is generated from the 1.2 V reference.
REF
DD
_2.5 pins. When using the internal reference, a 470 nF
V
2.5 V
2.048 V
1.25 V
2.5 V
2.048 V
1.25 V
REF
CM
Ranges
V
1.25 V
1.024 V
0.75 V
1.25 V
1.024 V
0.75 V
CM
Min
REF
V
2.05 V
2.276 V
2.55 V
1.75 V
1.976 V
2.25 V
CM
_2.5 pin and AGND to ensure
CM
CM
) that are dependent on
Max
minimum and V
DD
and V
REF
_1.2 and AGND and
2.5 V
2.5 V
Peak-to-Peak Signal
2.048 V
1.25 V
2.048 V
1.25 V
REF
REF
conditions.
_1.2 and
REF
CM
_1.2 pin.
REF
_1.2