Agilent Technologies 2712A, 2948 manual A3 Output Assembly Service Sheets 1 through General

Service Model 8904A

A3 OUTPUT ASSEMBLY (Service Sheets 1 through 4)

General

The A3 Output Assembly converts the digitally encoded waveform from the digital waveform synthesis integrated circuit (DWSIC) on the A2 Digital Assembly to a precise and useable analog signal. Instruments with Option 002 have a second Output Assembly (designated A10) which is identical to A3. For troubleshooting purposes the two Output Assemblies can be interchanged or compared node-for-node. Specifically, the Output Assembly contains:

0the digital-to-analog converter

0a track-and-hold circuit

0clean up filters

0amplifiers and attenuators

0reverse power protection

Digital-to-Analog Converter (Service Sheet 1)

IC U6 is a 12 bit Digital-to-Analog Converter (DAC). Its function is to generate current with a magnitude proportional to the weighted 12 bit input data from DWSIC via latches U4 and U5. New data is output to the latches at a 1.6777216 MHz clock rate. The latches hold the data until it is time to clock the data to the DAC. Clocking of the latches is delayed 60 ns by U16 for optimum synchronization of the DAC and the following Track-and-Hold Circuit.

The output of the DAC is converted to a voltage by the series combination of resistors R210, R224, and R221. R224 (DAC Level) is adjusted to give the correct waveform amplitude. Connector 54 is a convenient point for testing the DAC when jumper 5201 is moved to connect point 2 to point 3.

Track-and-Hold Circuit (Service Sheet 1)

After data is placed on the input of the DAC, the DAC requires 300 ns (that is, one-half clock period) settling before its output is valid. The Track-and-Hold circuit following the DAC stores (holds) the DAC’s previous valid output on a capacitor (C219 at the input of the Sampler Amplifier) for 300 ns while the DAC with its new input data settles. Then the DAC’s output is sampled and tracked for 300 ns.

The Track-and-Hold Circuit can be divided into three parts:

0Sampler Drive

0Sampler Switch

0Sampler Amplifier

Sampler Drive

The Sampler Drive must provide sufficientcurrent through transformer T1 to turn on hard the sampler diodes (U201A, B, C, and F) in the Sampler Switch. When the sampler diodes are on, the voltage at the DAC output transfers to capacitor C219 at the input of the Sampler Amplifier. Operation of the Sampler Drive is most easily seen by referring to the simplified schematic of Figure 8-9.

8-24Principles of Operation (A3)