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AD7740KRMZ Datasheet(PDF) 10 Page - Analog Devices

Part No. AD7740KRMZ
Description  3 V/5 V Low Power, Synchronous Voltage-to-Frequency Converter
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

AD7740KRMZ Datasheet(HTML) 10 Page - Analog Devices

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Temperature Sensor Application
The AD7740 can be used with an AD22100S temperature
sensor to give a digital measure of ambient temperature. The
output voltage of the AD22100S is proportional to the tempera-
ture times the supply voltage. It uses a single 5 V supply, and its
output swings from 0.25 V at –50
°C to 4.75 V at +150°C. By
feeding its output through the AD7740, the value of ambient
temperature is converted into a digital pulse train. See Figure 12.
0.1 F
10 F
0.1 F
10 F
Figure 12. Using the AD7740 with a Temperature Sensor
Due to its ratiometric nature this application provides an
extremely cost-effective solution. The need for an external pre-
cision reference is eliminated since the 5 V power-supply is used
as a reference to both the VFC and the AD22100S.
32 kHz Operation
The AD7740 oscillator circuit will not operate at 32 kHz. If
the user wishes to use a 32 kHz watch crystal, some additional
external circuitry is required. The circuit in Figure 13 is for a
crystal with a required drive of 1
µW. Resistors R1 and R2
reduce the power to this level.
Figure 13. 32 kHz Watch Crystal Circuit
Power Supply Bypassing and Grounding
In any circuit where accuracy is important, careful consideration
of the power supply and ground return layout helps to ensure
the rated performance. The printed circuit board housing the
AD7740 should be designed such that the analog and digital
sections are separated and confined to certain areas of the board.
To minimize capacitive coupling between them, digital and
analog ground planes should only be joined in one place, close
to the AD7740, and should not overlap.
Avoid running digital lines under the device, as these will couple
noise onto the die. The analog ground plane should be allowed
to run under the AD7740 to avoid noise coupling. The power
supply lines to the AD7740 should use as large a trace as pos-
sible to provide low impedance paths and reduce the effects of
glitches on the power supply line. Fast switching signals like
clocks should be shielded with digital ground to avoid radiating
noise to other parts of the board, and clock signals should never
be run near analog inputs. Avoid crossover of digital and analog
signals. Traces on opposite sides of the board should run at right
angles to each other. This reduces the effect of feedthrough
through the board. A microstrip technique is by far the best but
is not always possible with a double-sided board. In this technique,
the component side of the board is dedicated to the ground plane
while the signal traces are placed on the solder side.
Good decoupling is also important. All analog supplies should
be decoupled to GND with surface mount capacitors, 10
µF in
parallel with 0.1
µF located as close to the package as possible,
ideally right up against the device. The lead lengths on the by-
pass capacitor should be as short as possible. It is essential that
these capacitors be placed physically close to the AD7740 to
minimize the inductance of the PCB trace between the capacitor
and the supply pin. The 10
µF are the tantalum bead type and
are located in the vicinity of the VFC to reduce low-frequency
ripple. The 0.1
µF capacitors should have low Effective Series
Resistance (ESR) and Effective Series Inductance (ESI), such
as the common ceramic types, which provide a low imped-
ance path to ground at high frequencies to handle transient
currents due to internal logic switching. Additionally, it is ben-
eficial to have large capacitors (> 47
µF) located at the point
where the power connects to the PCB.

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