C
1.6 pF
h
Switch
ADCIN
28x DSP
Source
Signal
ac
C
5 pF
p
R
3.4 k
on
Ω
Sampling Capacitor (C ): 1.6 pF
h
Typical Values of the Input Circuit Components:
Parasitic Capacitance (C ): 5 pF
p
Switch Resistance (R ): 3.4 k
on
Ω
Source Resistance (R ): 50
S
Ω
R
S
Analog-to-Digital Converter (ADC)
900
SPRUHE8E – October 2012 – Revised November 2019
Copyright © 2012–2019, Texas Instruments Incorporated
Analog Subsystem
Table 10-2. TRIGxSEL Trigger Options (continued)
TRIGxSEL Bits
Trigger Source
Peripheral
11010
EPWM8SOCA
EPWM8
11011
EPWM8SOCB
11100
EPWM8SYNC
11101
EPWM9SOCA
EPWM9
11110
EPWM9SOCB
11111
EPWM9SYNC
10.3.3.1 ADC Acquisition (Sample and Hold) Window
External drivers vary in their ability to drive an analog signal quickly and effectively. Some circuits require
longer times to properly transfer the charge into the sampling capacitor of an ADC. To address this, the
ADC supports control over the sample window length for each individual SOC configuration. Each
ADCSOCxCTL register has a 6-bit field, ACQPS, that determines the sample and hold (S/H) window size.
The value written to this field is one less than the number of cycles desired for the sampling window for
that SOC. Thus, a value of 15 in this field will give 16 clock cycles of sample time. The minimum number
of sample cycles allowed is 7 (ACQPS=6). The total sampling time is found by adding the sample window
size to the conversion time of the ADC, 13 ADC clocks. Examples of various sample times are shown
below in
(1)
The total times are for a single conversion and do not include pipelining effects that increase the average speed over time.
Table 10-3. Sample timings with different values of ACQPS
ADC Clock
ACQPS
Sample Window
Conversion Time (13
cycles)
Total Time to Process
Analog Voltage
(1)
37.5MHz
6
187ns
347ns
534ns
37.5MHz
25
693ns
347ns
1040ns
30MHz
6
233ns
433ns
666ns
30MHz
25
867ns
433ns
1300ns
25MHz
6
280ns
520ns
800ns
25MHz
25
1040ns
520ns
1560ns
As shown in
, the ADCIN pins can be modeled as an RC circuit. With VREFLO connected to
ground, a voltage swing from 0 to 3.3v on ADCIN yields a typical RC time constant of 2ns.
Figure 10-12. ADCINx Input Model