AD4697 datasheet(87/107 Pages) AD

87 / 107 page

Data Sheet

AD4697/AD4698

APPLICATIONS INFORMATION

analog.com

Rev. 0 | 87 of 107

Implementing Two Effective Channel Sample

Rates

In multichannel data acquisition systems, the ADC may be moni-

toring a mix of higher frequency and lower frequency or dc type

signals. Channels with higher maximum input frequencies require

higher Nyquist frequencies, and therefore require higher effective

sample rates than channels with lower maximum input frequencies.

To maximize the effective sample rate for analog input channels

with higher frequency input signals, the channel sequence can be

designed to implement two different effective sample rates.

In a custom channel sequence that implements two effective

sample rates, each of the AD4697/AD4698 channels included in

the sequence are categorized as either high sample rate (HSR)

channels or low sample rate (LSR) channels. Figure 127 shows

a generalized channel sequence implementing HSR and LSR chan-

nels.

Figure 127. Sequence of HSR and LSR Inputs with Two Effective Sample

Rates

The full channel sequence in Figure 127 consists of a repeating sub

sequence of all HSR channels, followed by one LSR channel. The

sub sequences repeat until all LSR channels are sampled once,

and then the entire sequence starts again. As a result, the LSR

channels are sampled only once per sequence iteration, whereas

the HSR channels are sampled once for each LSR channel in the

sequence.

The number of HSR channels (NHSR) and the number of LSR

channels (NLSR) dictate their effective sample rates, as well as

the number of sequence positions required to implement the two

sample rates. The number of sequence positions required (NS)

follows the relation:

NS=NLSR× NHSR+1

(26)

where:

NHSR is the number of HSR inputs.

NLSR is the number of LSR inputs.

When the advanced sequencer is enabled, the maximum value of

NS is limited by the number of AS_SLOTn registers. When two-cy-

cle command mode or single-cycle command mode is enabled, NS

can be arbitrarily large.

Because the LSR channels are only sampled once per full se-

quence iteration, their effective sample rate (fS_LSR) is the sample

rate of the ADC core (set by fCNV) divided by NS as follows:

fS_LSR=fCNVNS

(27)

Because the HSR inputs are sampled once for each LSR input in

the sequence, the effective sample rate for the HSR inputs (fS_HSR)

is as follows:

fS_HSR=fCNV×NLSRNS

(28)

Table 30 shows an example where IN0, IN1, and IN2 are HSR

channels, and IN3, IN4, and the temperature sensor are LSR

channels.

Table 30. Sequence with Two Effective Channel Sample Rates

Sequence

Position

Input

Effective Sample Rate of Input

IN0

fCNV/4

IN1

fCNV/4

IN2

fCNV/4

IN3

fCNV/12

IN0

fCNV/4

IN1

fCNV/4

IN2

fCNV/4

IN4

fCNV/12

IN0

fCNV/4

IN1

fCNV/4

IN2

fCNV/4

Temperature sensor

fCNV/12

Note that implementing the sequence in Table 30 with the advanced

sequencer requires the following configuration settings:

►

STD_SEQ_EN = 0

►

NUM_SLOTS_AS = 10

►

TEMP_EN = 1

The first 11 advanced sequencer slots (AS_SLOT0 to AS_SLOT10)

are also programmed with the analog inputs listed in Table 30

because the temperature sensor is enabled via the TEMP_EN bit

instead of via the advanced sequencer slots.

Note that when using the advanced sequencer, the temperature

sensor cannot be assigned as an HSR channel because it cannot

be assigned with the AS_SLOTn registers. However, the tempera-

ture sensor can be included as an LSR channel by enabling it via

the TEMP_EN bit in the TEMP_CTRL register, as demonstrated in

Table 30.

AD4697 датащи(PDF) 87 Page - Analog Devices

AD4697 датащи(HTML) 87 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

датащи скачать

ссылки URL