.TH E1432_SET_ANTI_ALIAS_DIGITAL 3 E1432
.SH NAME
.nf
e1432_set_anti_alias_digital \- Enable/disable digital anti-alias filter
e1432_get_anti_alias_digital \- Get current state of digital anti-alias filter
.fi
.IX e1432_set_anti_alias_digital(3) 3
.IX e1432_get_anti_alias_digital(3) 3
.SH SYNOPSIS
.cS
SHORTSIZ16 e1432_set_anti_alias_digital(E1432ID hw, SHORTSIZ16 ID,
                                        SHORTSIZ16 state)
SHORTSIZ16 e1432_get_anti_alias_digital(E1432ID hw, SHORTSIZ16 ID,
                                        SHORTSIZ16 *state)
.cE
.SH DESCRIPTION
\fIe1432_set_anti_alias_digital\fR enables or disables the digital
anti-alias filter, of a single channel or group of channels \fIID\fR,
depending on the value given in \fIstate\fR.  The analog counterpart
to the digital anti-alias filter is controlled with another function,
\fIe1432_set_anti_alias_analog\fR.

\fIe1432_get_anti_alias_digital\fR returns the current state of the
digital anti-filter, of a single channel or group of channels
\fIID\fR, into a memory location pointed to by \fIstate\fR.

\fIhw\fR must be the result of a successful call to
\fIe1432_assign_channel_numbers\fR, and specifies the group of
hardware to talk to.

\fIID\fR is either the ID of a group of channels that was obtained
with a call to \fIe1432_create_channel_group\fR, or the ID of a single
channel.

\fIstate\fR determines the state of the digital anti-alias filter.

\fBE1432_ANTI_ALIAS_DIGITAL_ON\fR enables the digital anti-alias
filters.  These filters are elliptic IIR filters, which provide a very
flat pass-band and very good stop-band attenuation.  These are the
default filters.

\fBE1432_ANTI_ALIAS_DIGITAL_OFF\fR disables the digital
anti-alias filters.

\fBE1432_ANTI_ALIAS_DIGITAL_BES\fR enables the use of "Bessel"
anti-alias digital filters.  These filters are IIR digital filters
which have nearly linear phase and very little overshoot and ringing.
For this reason, these filters are good for doing time-domain
measurements.  However, these filters have several drawbacks when used
for frequency-domain measurements.  These filters cut the bandwidth
(the 3-dB point) of a measurement to roughly half of what it would be
with the elliptical filters, they provide only about 70 dB of alias
rejection, and they are not nearly as flat in the frequency domain as
the elliptical filters.

When using \fBE1432_ANTI_ALIAS_DIGITAL_BES\fR, the actual 3-dB
frequency of the data is roughly half of the span setting set by
\fIe1432_set_span\fR.  The effective data rate is still 2.56 times the
span setting.  It may be useful to increase the oversample rate by a
factor of two using the \fIe1432_set_decimation_oversample\fR.  This
will not change the 3-dB frequency of the data, but it will increase
the effective data rate to 5.12 times the span setting, doubling the
amount of data acquired in a given amount of time.

Changing the input digital filters while a measurement is running will
stop the measurement.

For input channels, disabling the digital filters results in data that
is not alias-protected in the frequency domain, and therefore this is
not usually a good idea for input channels.  For source channels,
disabling the digital filters results in an output signal that may
have high-frequency components in it.  For tach channels, there is no
digital filter and this function returns an error.

The E1432 and E1433 input SCAs support this parameter to control the
digital filtering done by the DSP chip in the SCAs.  However, note
that this enables and disables \fIonly\fR the decimation digital
filters.  It does not disable or change the digital filter that is
built into the Delta-Sigma ADCs (there is no way to disable this
filter and still have a working ADC), so there is still some digital
filtering applied to the data even when you attempt to completely
disable the digital filters.

On the E1432, this parameter applies to all channels on an SCA, and
can't be set separately for each channel.  This restriction is not
needed for the E1433.

Disabling the input digital filters is not usually a good idea because
aliasing can result.  Depending on the application, it may be better
to use \fBE1432_ANTI_ALIAS_DIGITAL_BES\fR, or the
\fIe1432_set_decimation_undersamp\fR function may be useful instead.
This function provides a different way to prevent use of the digital
anti-alias filters, by slowing down the ADC clock on the E1433.

.SH Source 20-Bit Mode

The E1432/3/4 Option 1D4 single-channel source and the E1434 source
normally have an interpolation digital filter immediately before the
output DAC.  This interpolation digital filter uses 16-bit input data,
so normally the arb source data path is only 16 bits wide.

This interpolation digital filter can be disabled using the
\fIe1432_set_anti_alias_digital\fR function, resulting in a 20-bit
signal path (the DAC can accept 20-bit data words).  When this
interpolation filter is disabled, the DAC output is not completely
alias protected by the 25.6 kHz analog output filter.  To get better
alias protection, the analog output filter must be switched to 6.4 kHz
(see \fIe1432_set_filter_freq\fR).  If the source is in random mode,
the selected span should be 6.4 kHz or less.  If the source is in arb
mode, either the span should be set to 6.4 kHz or less, or the arb
data should be band-limited by the host computer to 6.4 kHz or less.

There is only one 20-bit signal path for each 2-channel SCA in an
E1434, on the first of the two SCA channels.  Please refer to the
module block diagram in the E1434A Users Guide.

To enable the 20-bit mode, use the following function call sequence:
\fIe1432_set_active\fR, \fIe1432_set_source_mode\fR, \fIe1432_set_span\fR,
\fIe1432_set_anti_alias_digital\fR, \fIe1432_set_filter_freq\fR.

.SH "RESET VALUE"
After a reset, \fIstate\fR is set to \fBE1432_ANTI_ALIAS_DIGITAL_ON\fR.
.SH "RETURN VALUE"
Return 0 if successful, a (negative) error number otherwise.
.SH "SEE ALSO"
.na
e1432_set_anti_alias_analog, e1432_set_filter_freq,
e1432_set_decimation_undersamp
.ad
