Finally, t
S+H
is used to determine the minimum value to program into the ACQPS field of the ADCSOCxCTL
registers:
#%325
=
:
P
O
+
D
®
B
#&%%.-
;
F
1
Where the following parameters are provided by the ADC input model:
• n = ADC resolution (in bits)
• R
on
= ADC sampling switch resistance
• C
h
= ADC sampling capacitor
• C
p
= ADC parasitic pin capacitance for the channel
And the following parameters are dependent on the application design:
• settling error = tolerable settling error (in LSBs)
• R
s
= ADC driving circuit source impedance
• C
s
= ADC driving circuit source capacitance on ADC input pin
• f
ADCCLK
= ADC clock frequency
8.2.1.1.2 ACQPS Approximation Example for High Bandwidth Signals
For example, assuming the following parameters:
• n = 12-bits
• settling error =¼ LSB
• R
on
= 3400Ω
• C
h
= 1.6pF
• C
p
= 5pF
• R
s
= 56Ω
• C
s
= 2.2nF
• f
ADCCLK
= 30MHz
The time constant would be calculated as:
ì
=
:
56
3
+ 3400
3
;:
1.6
L(
;
+
:
56
3
;:
2200
L(
+ 5
L(
;
= 5.5
JO
+ 123.5
JO
= 129
JO
And the number of required time constants would be:
G
= ln
F
2
12
.5$
0.25
.5$
G
F
ln
l
2200
L(
+ 5
L(
1.6
L(
p
= 9.7
F
7.2 = 2.5
So the S+H time should be set to at least:
P
O
+
D
= 2.5
®
129
JO
= 322.5
JO
Finally, the minimum ACQPS value is calculated and rounded up to the nearest supported value:
#%325
=
:
322.5
JO
®
30
/*V
;
F
1 = 8.7
\
9
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)
516
TMS320x2806x Microcontrollers
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
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