Datasheet

ADuC7128/ADuC7129
Rev. 0 | Page 37 of 92
A
V
DD
C1
D
D
R1
C2
AV
DD
C1
D
D
R1
C2
06020-037
Figure 41. 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 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 42 and Figure 43 give an example of an ADC front end.
ADuC7128
ADC0
10
0.01µF
06020-038
Figure 42. Buffering Single-Ended/Pseudo Differential Input
ADuC7128
ADC0
V
REF
ADC1
06020-039
Figure 43. Buffering Differential Inputs
When no amplifier is used to drive the analog input, the source
impedance should be limited to values lower than 1 k. 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 reference can be used for the ADC. In
differential mode of operation, there are restrictions on the
common-mode input signal (V
CM
) that are dependent on
reference value and supply voltage used to ensure that the signal
remains within the supply rails. gives some calculated
V
Table 39
CM
minimum and V
CM
maximum values.
Table 39. V
CM
Ranges
AV
DD
V
REF
V
CM
Min V
CM
Max Signal Peak-to-Peak
2.5 V 1.25 V 2.05 V 2.5 V
2.048 V 1.024 V 2.276 V 2.048 V
3.3 V
1.25 V 0.75 V 2.55 V 1.25 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
3.0 V
1.25 V 0.75 V 2.25 V 1.25 V
TEMPERATURE SENSOR
The ADuC7128/ADuC7129 provide a voltage output from an
on-chip band gap reference proportional to absolute temperature.
The voltage output can also be routed through the front end
ADC multiplexer (effectively an additional ADC channel
input), facilitating an internal temperature sensor channel,
measuring die temperature to an accuracy of ±3°C.
The following is a code example of how to configure the ADC
for use with the temperature sensor:
int main(void)
{
float a = 0;
short b;
ADCCON = 0x20; // power-on the ADC
delay(2000);
ADCCP = 0x10; // Select Temperature Sensor as
// an input to the ADC
REFCON = 0x01;// connect internal 2.5V
// reference to Vref pin
ADCCON = 0xE4;// continuous conversion
while(1)
{
while (!ADCSTA){};
b = (ADCDAT >> 16);
// To calculate temperature in °C, use
the formula:
a = 0x525 - b;
// ((Temperature = 0x525 - Sensor
Voltage) / 1.3)
a /= 1.3;
b = floor(a);
printf("Temperature: %d oC\n",b);
}
return 0;
}