Sensoray 118 Uživatelský manuál stáhnout pdf (Strana 22)

Sensoray Model 118 Smart A/D™ Instruction Manual 20 Commands: Channel Configuration

6.2.3 SetFilter

Establishes a software single-pole low-pass filter for the

specified channel. The second command byte specifies a

filter factor, F, which may have any value from 0 to 255,

inclusive. The approximate relationship between F and

filter percentage, P, is:

All filter constants default to zero after a reset. This

effectively disables the software filters, thereby

maximizing the frequency response of all channels.

Applying a non-zero factor in electrically noisy

environments may help to reduce measurement noise.

Rather than computing the theoretical value of filter

constants, we recommend that you experiment with

different filter constants to find the best

response-time/noise-immunity tradeoff point.

Some applications require the use of a specific filter time

constant value. This function describes the relationship

between time constant and filter factor:

where: t is the time constant in seconds

F is the filter factor in the range 0 to 255

N is the number of active channels.

Driver

// Set the filter factor for the specified channel.

VOID SetFilter( USHORT BasePort, UCHAR Channel, UCHAR FiltFactor )

{

SendByte( BasePort, Channel | CMD_SET_FILT ); // send channel & opcode

SendByte( BasePort, FiltFactor ); // send filter factor

}

Example

// Set up channel 4 with a 12.5 percent low-pass filter.

SetFilter( MyBasePort, 4, (UCHAR)( 12.5 * 2.55 ) );

Command: (96 + Chan),(FilterFactor)

Response: None

2.55

F×=

N–

45ln

256

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