100
Chapter 5 Theory of Operation
Waveform DAC and Filters
5
Analog Circuitry
The analog circuitry begins at the Waveform DAC and continues to the
main output.
Sine, ramp, noise, and arbitrary waveforms pass directly from the
Waveform DAC to the main output circuitry. Square waves and pulses are
formed in the digital waveform and variable-rate level translation circuits.
Waveform DAC and Filters
See “A1 Waveform DAC, Filters, and Comparator Schematic” on page 201.
The 12-bit waveform DAC, U1701, is loaded with data by the
Synchronous Multiplexer of the digital circuitry. The most significant bit
of this data is inverted by U1401-B to convert the 2’s complement value
in memory to the offset-binary representation required by the DAC.
Data is clocked at a 200 MHz rate through differential clock inputs
WFDAC_CLK±. The waveform DAC clock is out of phase with the
LOGIC_CLK to provide ample setup and hold times for the data.
The DAC output, at pins 17 and 18, is centered at –250 mV and ranges
from 0 mV to –500 mV full scale.
Latching relay K1701 connects the DAC output to one of two filters:
•A 9
th
order elliptical filter with a cutoff frequency of 85 MHz.
This filter includes
sin(x)/x correction. It is used for continuous
sine and square waves.
•A 7
th
order linear-phase filter with a cutoff frequency of 50 MHz. It is
used for ramp, noise, and arbitrary waveforms. It is also used for sine
and square waves in burst mode.
Relay K1701 is driven by the SET_STEP and SET_SINE lines from U306.
The output of the selected filter is applied to amplifier U1703. U1703 has
a gain of 4.3 and the output is level shifted to center at ground potential.
The output ranges from –1.1 V to +1.1 V. U1702 uses VREF (+2.5 V) to
provide an output level appropriate to shift the waveform DAC output to
center around ground.
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