ADUC7121 Analog Devices, ADUC7121 Datasheet - Page 33

• Manufacturer Part Number: ADUC7121
• Description: Precision Analog Microcontroller, 12-Bit Analog I/O, ARM7TDMI MCU
• Manufacturer: Analog Devices

Specifications of ADUC7121

• Mcu Core: ARM7 TDMI
• Mcu Speed (mips): 40
• Sram (bytes): 8192Bytes
• Gpio Pins: 32
• Adc # Channels: 9

When no amplifier is used to drive the analog input, limit the
source impedance 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
An 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
dependent on the reference value and supply voltage used
to ensure that the signal remains within the supply rails.
Table 38 gives some calculated V
maximum values.
Table 38. V
AV
3.3 V
3.0 V
Table 39. REFCON MMR Bit Designations (Address = 0xFFFF0480, Default Value = 0x01)
Bit
7:1
2
1
0
DD
Description
Reserved.
BUF_VREF1/BUF_VREF2 is driven from the internal 2.5 V reference when set to 1.
Internal 2.5 V reference output enable.
Set by the user to connect the internal 2.5 V reference to the V
Cleared by the user to disconnect the reference from the V
reference source to it.
Internal 1.2 V reference output enable.
Set by the user to connect the internal 1.2 V reference to the V
Cleared by the user to disconnect the reference from the V
V
2.5 V
2.048 V
1.25 V
2.5 V
2.048 V
1.25 V
REF
CM
V
REF
Ranges
Figure 24. Buffering Differential Inputs
V
1.25 V
1.024 V
0.75 V
1.25 V
1.024 V
0.75 V
CM
Min
V
2.05 V
2.276 V
2.55 V
1.75 V
1.976 V
2.25 V
CM
CM
CM
Max
minimum and V
). These restrictions are
ADuC7121
ADC0
ADC1
Signal Peak-to-Peak
2.5 V
2.048 V
1.25 V
2.5 V
2.048 V
1.25 V
CM
Rev. 0 | Page 33 of 96
REF
REF
_2.5 pin. The V
_1.2 pin.
REF
REF
_2.5 pin.
_1.2 pin.
BAND GAP REFERENCE
The ADuC7121 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 refer-
ence is generated from a 1.2 V reference.
This internal reference also appears on the V
and V
0.47 μF must be connected between each external V
AGND to ensure stability and fast response during ADC
conversions. This reference can also be connected to the external
pin, BUF_VREF2, and used as a reference for other circuits in
the system.
The band gap reference also connects through buffers to the
BUF_VREF1 and the BUF_VREF2 pins. To damp the noise,
connect a minimum of 0.1 μF capacitor to these pins. The band
gap reference interface consists of an 8-bit REFCON MMR,
described in Table 39.
REF
REF
_1.2). When using the internal reference, a capacitor of
_2.5 pin should also be cleared to connect an external
REF
ADuC7121
pins (V
REF
pin and
REF
_2.5