Microchip Technology dsPIC30F - Transfer Function; A;D Accuracy;Error; Connection Considerations

© 2004 Microchip Technology Inc. DS70064C-page 17-47
Section 17. 10-bit A/D Converter
10-bit A/D
Converter
17
17.18 Transfer Function
The ideal transfer function of the A/D converter is shown in Figure 17-24. The difference of the
input voltages, (V
INH – VINL); is compared to the reference, (VREFH – VREFL).
• The first code transition occurs when the input voltage is (V
REFH – VREFL/2048) or 0.5 LSb.
• The 00 0000 0001 code is centered at (V
REFH – VREFL/1024) or 1.0 LSb.
• The 10 0000 0000 code is centered at (512*(V
REFH – VREFL)/1024).
• An input voltage less than (1*(V
REFH – VREFL)/2048) converts as 00 0000 0000.
• An input greater than (2045*(V
REFH – VREFL)/2048) converts as 11 1111 1111.
Figure 17-24: A/D Transfer Function
17.19 A/D Accuracy/Error
Refer to Section 17.26 “Related Application Notes”for a list of documents that discuss A/D
accuracy.
17.20 Connection Considerations
Since the analog inputs employ ESD protection, they have diodes to VDD and VSS. This requires
that the analog input must be between V
DD and VSS. If the input voltage exceeds this range by
greater than 0.3V (either direction), one of the diodes becomes forward biased and it may
damage the device if the input current specification is exceeded.
An external RC filter is sometimes added for anti-aliasing of the input signal. The R component
should be selected to ensure that the sampling time requirements are satisfied. Any external
components connected (via high-impedance) to an analog input pin (capacitor, zener diode, etc.)
should have very little leakage current at the pin.
10 0000 0010 (= 514)
10 0000 0011 (= 515)
01 1111 1101 (= 509)
01 1111 1110 (= 510)
01 1111 1111 (= 511)
11 1111 1110 (= 1022)
11 1111 1111 (= 1023)
00 0000 0000 (= 0)
00 0000 0001 (= 1)
Output
Code
10 0000 0000 (= 512)
(VINH – VINL)
V
REFL
VREFH – VREFL
1024
VREFH
VREFL +
10 0000 0001 (= 513)
512*(V
REFH – VREFL)
1024
VREFL +
1023*(V
REFH – VREFL)
1024
VREFL +