EVAL-AD7741/42EBZ【PDF 9/12 页】BOARD EVAL FOR AD7741/7742

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AD7741/AD7742

The digital data that represents the analog input voltage is con-

tained in the duty cycle of the pulse train appearing at the out-

put of the comparator. The output is a fixed-width pulse whose

frequency depends on the analog input signal. The input voltage

is offset internally so that a full-scale input gives an output fre-

quency of 0.45 f CLKIN and zero-scale input gives an output fre-

quency of 0.05 f CLKIN . The output allows simple interfacing to

either standard logic families or opto-couplers. The clock high

AD7741/AD7742

5M

TO OTHER

CIRCUITRY

period controls the pulsewidth of the frequency output. The

pulse is initiated by the edge of the clock signal. The delay time

between the edge of the clock and the edge of the frequency

CLKIN

C1

CLKOUT

C2

output is typically 9 ns. Figure 7 shows the waveform of this

frequency output.

After power-up, or if there is a step change in input voltage,

there is a settling time that must elapse before valid data is

obtained. This is typically 2 CLKIN cycles on the AD7742 and

10 CLKIN cycles on the AD7741.

f CLKIN

f OUT = f CLKIN /4

V IN = V REF /2

f OUT = f CLKIN /10

V IN = V REF /8

f OUT = f CLKIN *3/20

V IN = V REF /4

Figure 8. On-Chip Oscillator

The on-chip oscillator circuit also has a start-up time associated

with it before it oscillates at its correct frequency and correct

voltage levels. The typical start-up time for the circuit is 5 ms

(with a 6.144 MHz crystal).

The AD7741/AD7742 master clock appears on the CLKOUT

pin of the device. The maximum recommended load on this pin

is one CMOS load. When using a crystal to generate the AD7741/

AD7742 clock it may be desirable to then use this clock as the

clock source for the system. In this case it is recommended that

the CLKOUT signal be buffered with a CMOS buffer before

being applied to the rest of the circuit.

Reference Input

The AD7741/AD7742 performs conversion relative to an applied

reference voltage that allows easy interfacing to ratiometric

systems. This reference may be applied using the internal 2.5 V

6 T CLK

7 T CLK

bandgap reference. For the AD7741, this is done by simply

leaving REFIN/OUT unconnected. For the AD7742, REFIN is

AVERAGE f OUT IS f CLKIN *3/20 BUT THE ACTUAL PULSE STREAM

VARIES BETWEEN f CLKIN /6 AND f CLKIN /7

Figure 7. AD7741/AD7742 Frequency Output Waveforms

Clock Generation

As distinct from the asynchronous VFCs which rely on the stability

of an external capacitor to set their full-scale frequency, the

AD7741/AD7742 uses an external clock to define the full-scale

output frequency. The result is a more stable, more linear trans-

fer function and also allows the designer to determine the sys-

tem stability and drift based upon the external clock selected. A

crystal oscillator may also be used if desired.

The AD7741/AD7742 requires a master clock input, which may

be an external CMOS-compatible clock signal applied to the

CLKIN pin (CLKOUT not used). Alternatively, a crystal of the

correct frequency can be connected between CLKIN and

CLKOUT, when the clock circuit will function as a crystal

controlled oscillator. Figure 8 shows a simple model of the on-

chip oscillator.

REV. 0

–9 –

tied to REFOUT. Alternatively, an external reference, e.g.,

REF192 or AD780, may be used. For the AD7741, this is con-

nected to REFIN/OUT and will overdrive the internal refer-

ence. For the AD7742, it is connected directly to the REFIN

pin.

While the internal reference will be adequate for most applica-

tions, power supply rejection and overall regulation may be

improved through the use of an external precision reference.

The process of selecting an external voltage reference should

include consideration of drive capability, initial error, noise and

drift characteristics. A suitable choice would be the AD780 or

REF192.

Power-Down Mode

The low power standby mode is initiated by taking the PD pin

low, which shuts down most of the analog and digital circuitry.

This reduces the power consumption to 185 μ W max.

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