AD7769 데이터 시트보기 (PDF) - Analog Devices

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AD7769

LC2MOS Analog I/O Port

Analog Devices 

AD7769

CIRCUIT DESCRIPTION

Analog Inputs and Outputs

The AD7769 provides the analog-to-digital and digital-to-analog

conversion functions required between the microcontroller and

the servo power amplifier in digital servo systems. It is intended

primarily for closed loop head positioning in Winchester disk

drives, but may also be used for microstepping in drives with

stepper motor head positioning or other servo applications. The

AD7769 contains a high speed, 8-bit, sampling ADC with two

input channels and two 8-bit DACs with output buffer amplifi-

ers. A unique feature of the AD7769 is the input and output sig-

nal conditioning circuitry that allows the analog input and

output voltages to be referred to a point other than analog

ground. The input range and offset of the ADC, the output

swing and offset of the DACs may be adjusted independently by

the application of ground-referenced, positive control voltages,

VBIAS (ADC), VSWING (ADC), VBIAS (DAC) and VSWING (DAC).

Thus, for example, the peak-to-peak output swing of the DACs

could be set to 3 V above and 3 V below a bias voltage of 5 V.

Figures 5 and 6 show the transfer functions of the ADC

and DACs and their relationship to VBIAS and VSWING. The

midpoint code of the ADC, 80 Hex (10000000 Binary), occurs

at an input voltage equal to VBIAS. The input FSR of the ADC is

equal to 2 VSWING, so that the Plus Full-Scale code transition

(FE to FF Hex) occurs at a voltage equal to VBIAS + VSWING

–1.5 LSBs and the Minus Full-Scale code transition (01 to 00

Hex) occurs at a voltage VBIAS – VSWING +0.5 LSBs. The

transfer function of the DACs bears a similar relationship to

VBIAS and VSWING. The DAC output voltage for code 80 Hex

(10000000 binary) is equal to VBIAS, while FF Hex (11111111

binary) gives an output voltage of VBIAS + VSWING –1 LSB

(Plus Full-Scale) and 00 Hex gives an output voltage of VBIAS –

VSWING (Minus Full-Scale).

The ability to refer input and output signals to some voltage

other than ground is of particular importance in disk drive ap-

plications. Typically, only +5 V digital and +12 V analog supply

voltages are available, and the analog signals are often referred

to a voltage around half the analog supply.

Driving the Analog Inputs and Reference Inputs

The analog inputs, VINA and VINB, must be driven from low

output impedance sources, such as from op amps. In addition,

VBIAS (ADC) must be driven from a similar type low impedance

source (e.g., voltage reference).

Op amps are not required to drive the VSWING (ADC), VBIAS

(DAC) and VSWING (DAC) inputs as these are high impedance

inputs (200 nA typical input current) that feed into on-chip

buffer amplifiers. The reference voltages for these inputs can be

derived using suitable resistor divider networks.

The analog reference available in the disk drive system can be

used to set the bias voltage of the AD7769, and could also be at-

tenuated to provide the reference for the input and output swing

as shown in Figure 7. The same bias voltage would generally

(though not necessarily) be used for the ADC and the DACs,

though the input and output ranges might be different.

Figure 5. ADC Transfer Function

Figure 6. DAC Transfer Function

Figure 7. Typical Analog Connections to the AD7769

ADC Conversion Cycle

Figure 8 shows the operating waveforms for a conversion cycle.

On the rising edge of WR, the conversion cycle starts with the

acquisition and tracking of the selected ADC channel, VINA or

VINB. The analog input voltage is held 50 ns (typically) after the

fourth falling edge of the input CLK following a conversion

start. If tD in Figure 8 is greater than 150 ns, then the falling

edge of the input CLK will be seen as the first falling clock edge.

If tD is less than 150 ns, the first falling clock edge to be recog-

nized will not occur until one cycle later.

–8–

REV. A

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