8-Bit Octal, 4-Quadrant Multiplying, CMOS TrimDAC
The AD8842 provides eight general purpose digitally controlled voltage adjustment devices. The TrimDAC® capability allows replacement of the mechanical trimmer function in new designs. The AD8842 is ideal for ac or dc gain control of up to 50 kHz bandwidth signals. The four-quadrant multiplying capability is useful for signal inversion and modulation often found in video vertical deflection circuitry.
Internally the AD8842 contains eight voltage output digital-to-analog converters, each with separate voltage inputs. A new current conveyor amplifier design performs the four-quadrant multiplying function with a single amplifier at the output of the current steering digital-to-analog converter. This approach offers an improved constant input resistance performance versus previous voltage switched DACs used in TrimDAC circuits, eliminating the need for additional input buffer amplifiers.
Each DAC has its own DAC register that holds its output state. These DAC registers are updated from an internal serial-to-parallel shift register that is loaded from a standard 3-wire serial input digital interface. Twelve data bits make up the data word clocked into the serial input register. This data word is decoded where the first 4 bits determine the address of the DAC register to be loaded with the last 8 bits of data. A serial data output pin at the opposite end of the serial register allows simple daisy chaining in multiple DAC applications without additional external decoding logic.
The AD8842 consumes only 95 mW from ±5 V power supplies. For single 5 V supply applications consult the DAC-8841. The AD8842 is pin compatible with the 1 MHz multiplying band-width DAC8840. The AD8842 is available in 24-pin plastic DIP and surface mount SOL-24 packages.
- Automatic Adjustment
- Trimmer Replacement
- Vertical Deflection Amplitude Adjustment
- Waveform Generation and Modulation