The AD7776, AD7777 and AD7778 are a family of high speed, multichannel, 10-bit ADCs primarily intended for use in R/W head positioning servos found in high density hard disk drives. They have unique input signal conditioning features that make them ideal for use in such single supply applications. By setting a bit in a control register within both the four-channel version, AD7777, and the eight-channel version, AD7778, the
input channels can either be independently sampled or any two
channels of choice can be simultaneously sampled. For all versions
the specified input signal range is of the form VBIAS ±
VSWING. However, if the RTN pin is biased at, say, 2 V the
analog input signal range becomes 0 V to +2 V for all input channels. This is covered in more detail under the section Changing the Analog Input Voltage Range. The voltage VBIAS is the offset of the ADC's midpoint code from ground and is supplied either by an onboard reference available to the user (REFOUT) or by an external voltage reference applied to REFIN. The full-scale range (FSR) of the ADC is equal to 2 VSWING where VSWING is nominally equal to REFIN/2. Additionally, when placed in the half-scale conversion mode, the
value of REFIN is converted. This allows the channel offset(s) to be measured.
Control register loading and ADC register reading, channel select and conversion start are under the control of the µP. The twos complemented coded ADCs are easily interfaced to a standard 16-bit MPU bus via their 10-bit data port and standard microprocessor control lines.
The AD7776/AD7777/AD7778 are fabricated in linear compatible CMOS (LC2 MOS), an advanced, mixed technology process that combines precision bipolar circuits with low power CMOS logic. The AD7776 is available in a 24-pin SOIC package; the AD7777 is available in both 28-pin DIP and 28-pin SOIC packages; the AD7778 is available in a 44-pin PQFP package.
