DAC CIRCUITRY

We now need convert the 7-bit latched data to an analogue signal. These are a number of 8-bit DAC I.C.'s suitable for the job, such as the DAC08 (used in the prototype), DAC0801 or MC1408. Since we are only producing 7-bit data then we connect the data into the upper 7 bits of the DAC, connecting the DAC's data bit 0 to 0V.

Whichever DAC is chosen, it is usually necessary to buffer the DAC's output. This job is often performed by a high speed op-amp. These op-amps can be troublesome particularly where poor wiring / grounding can cause problems with instability (for example "Vero Board" type of prototype P.C.B. is not well suited to high speed circuitry).

DAC Buffer and Sync Mixer [4K]Instead it was found that simpler transistor circuitry provided good performance. The circuit shown opposite uses a single small-signal PNP transistor to both buffer the DAC signal and mix it with the composite synch pulses.


 

The composite video now needs to be connected to a modulator. As this may represent a low impeadance, an additional transistor emitter follower is used to buffer the composite video signal and provide a 75-80 ohm source impeadance.

NEXT PAGE : Download Schematic and Software


Page copyright
J.Evans 2002
CLICK HERE TO RETURN TO
THE TEST CARD PAGE
Last updated
28th July 2002