8.2.1.2 ACQPS Approximation for Low Bandwidth Signals
Signals that are sampled infrequently and are tolerant of low-pass filtering (such as ambient temperature
sensors) can be treated as low bandwidth signals. A large C
s
that is sized to be much larger than C
h
will allow
the ADC to sample the R
s
C
s
filtered signal quickly at the expense of increased phase delay. C
h
will receive
the bulk of its charge from C
s
during the S+H window, and C
s
will recover its charge through R
s
between ADC
samples.
8.2.1.2.1 ACQPS Approximation Equations for Low Bandwidth Signals
The desired settling accuracy will determine the value of C
s
:
%
O
=
%
D
l
2
J
OAPPHEJC
ANNKN
p
The desired recovery time and acceptable charge error will determine the value of R
s
:
4
O
=
P
NA?KRANU
J
ì
®
%
O
With this configuration, C
s
acts as the effective source signal, which simplifies the equations for calculating (k)
and (τ) as follows:
ì
=
4
KJ
®
%
D
G
= ln
l
2
J
OAPPHEJC
ANNKN
p
So the total S+H time (t
S+H
)should be set to at least:
P
O
+
D
=
G
®
ì
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)
• t
recovery
= amount of time between ADC samples
• n
τ
= number of RC time constants that comprise t
recovery
• R
s
= ADC driving circuit source impedance
• C
s
= ADC driving circuit source capacitance on ADC input pin
• f
ADCCLK
= ADC clock frequency
The selection of n
τ
will determine how much the voltage on C
s
is able to recover between samples. An
insufficient amount of recovery time will introduce a droop error by way of an undercharged C
s
.
shows the relationship between n
τ
and the estimated droop error.
Analog-to-Digital Converter (ADC)
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
TMS320x2806x Microcontrollers
517
Copyright © 2022 Texas Instruments Incorporated
Summary of Contents for TMS320 2806 Series
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