参数资料
型号: EVAL-ADUC7121QSPZ
厂商: Analog Devices Inc
文件页数: 27/96页
文件大小: 0K
描述: BOARD EVALUATION FOR ADUC7121
设计资源: ADUC7121 Gerber Files
ADUC7121 Schematic
标准包装: 1
系列: MicroConverter® ADuC7xxx
类型: MCU
适用于相关产品: ADUC7121
所含物品: 板,CD
Data Sheet
ADuC7121
Rev. B | Page 33 of 96
Figure 25. Buffering Differential Inputs
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 (VCM). These restrictions are
dependent on the reference value and supply voltage used
to ensure that the signal remains within the supply rails.
Table 38 gives some calculated VCM minimum and VCM
maximum values.
Table 38. VCM Ranges
AVDD
VREF
VCM Min
VCM Max
Signal Peak-to-Peak
3.3 V
2.5 V
1.25 V
2.05 V
2.5 V
2.048 V
1.024 V
2.276 V
2.048 V
1.25 V
0.75 V
2.55 V
1.25 V
3.0 V
2.5 V
1.25 V
1.75 V
2.5 V
2.048 V
1.024 V
1.976 V
2.048 V
1.25 V
0.75 V
2.25 V
1.25 V
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 VREF pins (VREF_2.5
and VREF_1.2). When using the internal reference, a capacitor of
0.47 F must be connected between each external VREF pin and
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.
Table 39. REFCON MMR Bit Designations (Address = 0xFFFF0480, Default Value = 0x01)
Bit
Description
7:1
Reserved.
2
BUF_VREF1/BUF_VREF2 is driven from the internal 2.5 V reference when set to 1.
1
Internal 2.5 V reference output enable.
Set by the user to connect the internal 2.5 V reference to the VREF_2.5 pin.
Cleared by the user to disconnect the reference from the VREF_2.5 pin. The VREF_2.5 pin should also be cleared to connect an external
reference source to it.
0
Internal 1.2 V reference output enable.
Set by the user to connect the internal 1.2 V reference to the VREF_1.2 pin.
Cleared by the user to disconnect the reference from the VREF_1.2 pin.
09492-
024
ADuC7121
ADC0
VREF
ADC1
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