ADUM7703 datasheet(19/22 Pages) AD

19 / 22 page

Data Sheet

ADuM7703

Rev. 0 | Page 19 of 22

The input filter configuration for the ADuM7703 is not limited

to the low-pass structure shown in Figure 29. The differential RC

filter configuration shown in Figure 30 also achieves excellent

performance. Recommended values for the resistors and

capacitor are 22 Ω and 47 pF, respectively.

VIN–

VIN+

ADuM7703

Figure 30. Differential RC Filter Network

DIGITAL FILTER

The output of the ADuM7703 is a continuous digital bit stream.

To reconstruct the original input signal information, this output

bit stream must be digitally filtered and decimated. A sinc filter

is recommended due to simplicity of the filter. A sinc3 filter is

recommended because the filter is one order higher than that of

the ADuM7703 modulator, which is a second-order modulator.

The type of filter selected, the decimation rate, and the modulator

clock used determines the overall system resolution and

throughput rate. The higher the decimation rate, the greater the

system accuracy, as shown in Figure 31. However, there is a

trade-off between accuracy and throughput rate and, therefore,

higher decimation rates result in lower throughput solutions.

Note that for a given bandwidth requirement, a higher MCLKIN

frequency can allow higher decimation rates to be used, resulting

in higher SNR performance.

100

DECIMATION RATE (MHz)

1000

fIN = 1kHz

Figure 31. SNR vs. Decimation Rate of Sinc3 Filter Order

A sinc3 filter is recommended for the ADuM7703. This filter

can be implemented on a field programmable gate array

(FPGA) or a digital signal processor (DSP). Equation 1

describes the transfer function of a sinc filter.



() =













(1)

where:

Z is the sample.

DR is the decimation rate.

N is the sinc filter order.

The throughput rate of the sinc filter is determined by the

modulator clock and the decimation rate selected.

Throughput = MCLK/DR

(2)

where MCLK is the modulator clock frequency.

As the decimation rate increases, the data output size from

the sinc filter increases. The output data size is expressed in

Equation 3. The 16 most significant bits are used to return a

16-bit result.

Data Size = N × log2 DR

(3)

For a sinc3 filter, the −3 dB filter response point can be derived

from the filter transfer function, Equation 1, and is 0.262 times

the throughput rate. The filter characteristics for a third-order

sinc filter are summarized in Table 14.

Table 14. Sinc3 Filter Characteristics for 20 MHz MCLKIN

Decimation Ratio (DR)

Throughput Rate (kHz)

Output Data Size (Bits)

Filter Response (kHz)

625

163.7

312.5

81.8

128

156.2

40.9

256

78.1

20.4

512

39.1

10.2

ADUM7703 Datasheet(PDF) 19 Page - Analog Devices

ADUM7703 Datasheet(HTML) 19 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

Datasheet Descarga

Enlace URL