
LTC6602
21
6602fc
Figure 14. Mute Function Recovery Time
APPLICATIONS INFORMATION
Mute Function
The LTC6602 features a mute function which is asserted
by pulling MUTE (Pin 20) to ground. This breaks the signal
path that leads from the input pins to the filter networks,
attenuating the input signal by at least 20dB. The mute
function can be used to protect the filter inputs from large
transients. The filter clock continues to run when the filter
is muted, allowing for a fast recovery time when MUTE
is deasserted. Typically, the recovery time is less than
5s, as shown in Figure 14. When the mute function is
asserted, the differential input impedance becomes very
high, but the common mode input impedance to ground
remains the same. This keeps the input common mode
voltage stable when muted, even when the inputs are
AC coupled. Connecting GAIN0(D0) to MUTE allows for
serial control of the mute function. MUTE has an internal
pull-up to V+D.
A ground plane should be used. Noisy signals should be
isolated from the filter input pins.
TheoutputDCoffsettypicallychangeslessthan±2mVwhen
theclockfrequencyvariesfrom24.705MHzto90MHz.The
offset is measured by connecting the inputs to VOCM and
measuring the differential voltage at the filter’s output.
Aliasing
Aliasingisaninherentphenomenonofsampleddatafilters.
Significant aliasing only occurs when the frequency of the
input signal approaches the sampling frequency or mul-
tiples of the sampling frequency. The ratio of the LTC6602
input sampling frequency to the clock frequency, fCLK, is
determined by the state of control bit LPF1. If LPF1 is set
to ‘0’, the input sampling frequency is equal to fCLK/3. If
LPF1 is set to ‘1’, the input sampling frequency is equal to
fCLK.Inputsignalswithfrequenciesneartheinputsampling
frequency will be aliased to the passband of the filter and
appear at the output unattenuated.
A simple LC anti-aliasing filter is recommended at the filter
inputs to attenuate frequencies near the input sampling
frequencythatwillbealiasedtothepassband.Forexample,
if the clock frequency is set to 90MHz and the lowpass
cutoff frequency of the filter is set to it’s maximum (LPF1
= ‘1’), the lowest frequency that would be aliased to the
passband would be fCLK – fCUTOFF , i.e. 90MHz – 900kHz =
89.1MHz. In order to attenuate this frequency by 40dB, an
LCfilterwithacutofffrequencyof8.91MHzorlowerwould
be required at the filter inputs. The capacitor connected
between the LTC6602 filter inputs should be at least 150pF
to provide sufficient charge to the input sampler. If there
is no anti-aliasing filter, the LTC6602 filter inputs should
be driven by a low impedance source (<100).
Wideband Noise
The wideband noise of the filter is the RMS value of the
device’soutputnoisespectraldensity.Thewidebandnoise
voltage is used to determine the operating signal-to-noise
ratio at a given distortion level. The wideband noise is
nearly independent of the value of the clock frequency
and excludes the clock feedthrough. Most of the wideband
noise is concentrated in the filter passband and cannot be
removed with post filtering.
6602 F14
MUTE (2V/DIV)
VOUT (1V/DIV)
4s/DIV
Clock Feedthrough
Clock feedthrough is defined as the RMS value of the clock
frequency and its harmonics that are present at the filter’s
output. The clock feedthrough is measured with +INA and
–INA (or +INB, –INB) tied to VOCM and depends on the PC
board layout and the power supply decoupling. The clock
feedthrough can be reduced with a simple RC post filter.
DC Offset
The output DC offset of the LTC6602 is less than ±15mV.
To obtain optimum DC offset performance, appropriate
PC board layout techniques should be used. The filter
IC should be soldered to the PC board. The power sup-
plies should be well decoupled including 0.1F ceramic
capacitors from V+D (Pin 16) and V+A (Pin 2) to ground.