AD7769 datasheet(14/16 Pages) AD

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AD7769

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REV. A

Increased Resolution DAC Output

Since both VBIAS (DAC) and VSWING (DAC) are common to

both output channels, the full-scale output voltages of both

channels are nominally identical. However, by adding an exter-

nal op amp and scaling resistors, it is possible to attenuate the

full-scale output voltage of one (or both) of the DAC outputs to

effectively increase the output voltage resolution. Figure 25

shows channel A being attenuated using a resistor scaling of

10:1. The attenuated output voltage, VOUTA', is

VOUTA' = VBIAS + (VSWING/10)(2DA–1).

The output voltage of Channel B remains at

VOUTB = VBIAS + VSWING (2DB–1).

DA and DB are fractional representations of the DAC input

codes, e.g., DA = NA/256 and DB = NB/256. For example, with a

VSWING voltage level of 2 V, the Channel B output span is 4 V

with an LSB size of 15.6 mV and (attenuated) Channel A out-

put span is 400 mV with an LSB size of 1.56 mV. Changing the

resistor scaling in Figure 25 obviously changes the attenuated

full-scale output.

Figure 25. Increasing the DAC Output Voltage Resolution

A single change to the circuit Figure 25 allows the two DAC

outputs to be combined to provide a single analog output with

resolution beyond the standard 8-bits. Figure 26 shows the rear-

ranged circuit. The composite output, VOUT, is

VOUT = VOUTB + (VSWING/10)(2DA–1)

VOUT = VBIAS + VSWING (2DB–1) + (VSWING/10) (2DA–1).

Figure 26. Combined VOUTA, VOUTB Circuit

DAC A can be programmed to produce an interpolation func-

tion between the 8-bit steps of DAC B to allow, for example,

very smooth velocity profile waveforms to be generated.

Servo Offset Facility

Most dedicated servo disk drives offer an offset facility whereby

some small voltage is injected into the track-following loop. The

purpose of the offset is to move the head to the right or left of its

current on-track position to permit reading of off-track data.

The circuit is shown in Figure 27. With the 10:1 resistor scaling

used in the circuit the output voltage, VOUT, is

VOUT = VPE + (VSWING/10) (2DA–1).

Figure 27. Servo Offset Facility

With no offset added, VOUT = VPE, where VPE is the position

error voltage which the servo loop normally drives to its zero

level, VBIAS. When an offset voltage is supplied by DAC A, the

action of the servo is to move the head away from its current

on-track position until the position error voltage is equal and

opposite to the offset voltage. The position of the head about

the track centre is thus programmable.

Programmable Full-Scale Range

The output voltage span of both DACs is determined by the

VSWING (DAC) voltage level. This is normally supplied from

some fixed voltage source. However, it is possible to use one of

the DAC channels to generate a programmable VSWING voltage

level. The remaining channel will thus have a full-scale range

and LSB size which is software programmable. This circuit is

shown in Figure 28 where VOUTB is used in an implicit feedback

loop to generate a programmable swing voltage, VSWING (DAC),

for the AD7769 from an external fixed input swing voltage,

VSWING. Using the 5:1 resistor scaling shown in Figure 28, the

expression for the AD7669 input swing voltage is

VSWING (DAC) =

V SWING

1–

2DB –1

()

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