Maxim-integrated MAX31782 Bedienungsanleitung PDF herunterladen (Seite 46)

Maxim Integrated 6-3

MAX31782 User’s Guide

Revision 0; 8/11

6.1.2 Conversion Sequencing

The MAX31782 ADC performs a user-defined sequence of up to eight conversions. Each conversion in a sequence

is set up using one of the eight ADC configuration registers. The configuration registers are accessed by writing to

the ADDATA register when ADST.ADCFG = 1. The configuration register pointed to by ADDATA is selected using the

ADIDX bits in the ADST register. The individual configuration registers allows each of the conversions in a sequence to

select from the following options. For more information, see the 6.2.4 ADC Data and Configuration Register (ADDATA)

description.

• External temperature or voltage conversion

• Full-scale range

• Extended acquisition enable

• ADC conversion data alignment (left or right)

• Differential or single-ended conversion

• ADC channel selection

A sequence is set up in the ADADDR register by defining the starting conversion configuration address (ADSTART) and

an ending conversion configuration address (ADEND). The configuration start address designates the configuration

sequence conversion, where the starting and ending configuration address is the same. The configuration registers

can be viewed as a circular register array where ADSTART does not have to be less than ADEND. For example, if

ADSTART = 1 and ADEND = 5, the sequence of conversions would be configurations 1, 2, 3, 4, 5. If ADSTART = 5 and

ADEND = 1, the sequence of conversions would be configurations 5, 6, 7, 0, 1.

The ADC has two conversion sequence modes, single and continuous, which is set by the ADCONT bit. The start

conversion bit (ADCONV) is used to start all conversions. In single sequence mode (ADCONT = 0), ADCONV remains

set until the ADC has finished conversion on the last channel in the sequence. In continuous mode (ADCONT = 1), the

ADCONV bit remains set until the continuous mode is stopped. Writing a 0 to the ADCONV bit stops the ADC operation

at the completion of the current ADC conversion. Writing a 1 to the ADCONV bit when ADCONV bit is already set to 1

is ignored by the ADC controller.

6.1.3 ADC Conversion Time

The ADC clock is derived from the system clock with divide ratio defined by ADC clock divider bits (ADCN.

ADCCLK[2:0]). Each sample takes 17 ADC clock cycles to complete. Three of the 17 ADC clock cycles are used for

sample acquisition, and the remaining 14 clocks are used for data conversion. The ADC automatically reads each mea-

surement twice and outputs the average of the two readings. This makes the resulting time for one complete conversion

34 ADC clock cycles.

Knowing this, it is possible to calculate the fastest ADC sample rate. The fastest ADC clock is:

ADC Clock = Sysclk/16 = 4MHz/16 = 250kHz

One conversion requires 34 ADC clocks:

Sample Rate = ADC Clock/34 Clocks = 250kHz/34 = 7.353ksps, or 136Fs per sample

The ADC has an internal power management system that automatically shuts down the ADC when conversions are

complete by clearing ADCONV to 0. After being shut down, the ADC begins conversions again when the ADCONV bit

is set to 1 again. After ADCONV is set to 1, the ADC requires 20 ADCCLK cycles to set up and power-up prior to begin-

ning the first conversion of the sequence.

In applications where extending the acquisition time is desired, the user can make use of the ADC acquisition extension

bits (ADCN.ADACQ[3:0]). When the ADC acquisition extension is enabled (ADACQEN = 1), the sample is acquired

over a prolonged period. The extended acquisition time is determined by ADACQ[3:0] and the ADC clock divider used.

Table 6-1 shows the extended acquisition time in terms of ADC clocks at different ADACQ[3:0] and clock divider set-

tings. The total acquisition time, ACQ, is the extended acquisition time (ADACQ, as listed in Table 6-1) plus three ADC

clock cycles. Figure 6-2 shows the clocking required for one conversion.

1 2 ... 41 42 43 44 45 46 47 48 49 50 51 ... 222 223

MAX31782 User’s Guide 1

Kommentare zu diesen Handbüchern

Keine Kommentare

Maxim-integrated MAX31782 Bedienungsanleitung Seite 46

Kommentare zu diesen Handbüchern

Verwandte Produkte und Handbücher für Hardware Maxim-integrated MAX31782