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Table 8-2. Estimated Droop Error from n
τ
Value
n
τ
Droop Error
(% of Settling Error)
Droop Error for C
s
sized to
¼ LSB Settling Error (LSB)
Total Error (Droop Error +
¼ LSB Settling Error)
0.25
352%
0.88 LSB
1.13 LSB
0.50
154%
0.39 LSB
0.64 LSB
0.75
90%
0.23 LSB
0.48 LSB
1.00
58%
0.15 LSB
0.40 LSB
2.00
16%
0.04 LSB
0.29 LSB
3.00
5%
0.01 LSB
0.26 LSB
4.00
2%
0.01 LSB
0.26 LSB
5.00
1%
0.00 LSB
0.25 LSB
8.2.1.2.2 ACQPS Approximation Example for Low Bandwidth Signals
For example, assuming the following parameters:
• n = 12-bits
• settling error =¼ LSB
• t
recovery
= 1 ms
• n
τ
= 2
• R
on
= 3400 Ω
• C
h
= 1.6 pF
• C
p
= 5 pF
• f
ADCCLK
= 30 MHz
The minimum source capacitance would be calculated and rounded up to a common value:
%
O
= 1.6
L(
F
2
12
.5$
0.25
.5$
G
= 26.2
J(
\
33
J(
Then the maximum source resistance would be calculated and rounded down to a common value:
4
O
=
1
IO
2
®
33
J(
= 15.2
G
3
\
12k
3
Now the time constant (τ) and multiple (k) can be calculated as:
ì
= 3400
3
®
1.6
L(
= 5.4
JO
G
= ln
F
2
12
.5$
0.25
.5$
G
= 9.7
So the S+H time should be set to at least:
P
O
+
D
= 9.7
®
5.4
JO
= 52.4
JO
Finally, the minimum ACQPS value is calculated and rounded up to the nearest supported value:
#%325
=
:
52.4
JO
®
30
/*V
;
F
1 = 0.6
\
6
While this gives a rough estimate of the required acquisition window, a better method would be to setup a circuit
with the ADC input model, a model of the source impedance/capacitance, and any board parasitics in SPICE (or
similar software) and simulate to verify that the sampling capacitor settles to the desired accuracy.
Analog-to-Digital Converter (ADC)
518
TMS320x2806x Microcontrollers
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
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