AD7890 datasheet(23/29 Pages) AD

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AD7890

Rev. C | Page 22 of 28

In the scheme outlined here, the user does not have to worry

about monitoring the end of conversion. Once conversion is

complete, the AD7890 takes care of transmitting back its

conversion result to the processor. Once the 16 bits of data have

been received by the processor into its serial shift register, it

generates an internal interrupt. Since the RFS pin and the TFS

pin are connected together, data is transmitted to the control

conversion result. The user just has to ensure that the word to

be written to the AD7890 control register is set up prior to the

end of conversion. As part of the interrupt routine, which

recognizes that data has been read in, the processor can set up

the data it is going to write to the control register next time around.

AD7890

SMODE

RFS

TFS

DATA OUT

DATA IN

SCLK

TMS320C25/C30

FSR

FSX

CLKX

CLKR

CLK IN

CLK INPUT

Figure 18. AD7890 to TMS320C25/30 Interface

ANTIALIASING FILTER

The AD7890 provides separate access to the multiplexer and

ADC via the MUX OUT pin and the SHA IN pin. One of the

reasons for this is to allow the user to implement an antialiasing

filter between the multiplexer and the ADC. Inserting the

antialiasing filter at this point has the advantage that one

antialiasing filter can suffice for all eight channels rather than a

separate antialiasing filter for each channel if they were to be

placed prior to the multiplexer.

The antialiasing filter inserted between the MUX OUT pin and

the SHA IN pin is generally a low-pass filter to remove high

frequency signals which could possibly be aliased back in-band

during the sampling process. It is recommended that this filter

is an active filter, ideally with the MUX OUT pin of the AD7890

driving a high impedance stage and the SHA IN pin of the part

being driven from a low impedance stage. This removes any

effects from the variation of the part’s multiplexer on-resistance

with input signal voltage, and removes any effects of a high

source impedance at the sampling input of the track/hold. With

an external antialiasing filter in place, the additional settling

time associated with the filter should be accounted for by using

a larger capacitance on CEXT.

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