AD7740KRMZ Analog Devices Inc, AD7740KRMZ Datasheet - Page 9

• Manufacturer Part Number: AD7740KRMZ
• Description: IC CONV SYNC VOLT-FREQ 8-MSOP
• Manufacturer: Analog Devices Inc
• Type: Voltage to Frequencyr

Specifications of AD7740KRMZ

• Frequency - Max: 500kHz
• Full Scale: ±50ppm/°C
• Linearity: ±0.012%
• Mounting Type: Surface Mount
• Package / Case: 8-TSSOP, 8-MSOP (0.118", 3.00mm Width)
• Frequency: 1MHz
• Full Scale Range: 500kHz
• Linearity %: 1%
• Supply Voltage Range: 3V To 5.25V
• Digital Ic Case Style: SOIC
• No. Of Pins: 8
• Msl: MSL 3 - 168 Hours
• Frequency Max: 1MHz
• Converter Function: VFC
• Full Scale Frequency: 1000
• Power Supply Requirement: Single
• Single Supply Voltage (max): 5.25V
• Single Supply Voltage (min): 3V
• Dual Supply Voltage (typ): Not RequiredV
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temperature (min): 0C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Commercial
• Package Type: MSOP
• Converter Type: Voltage/Frequency
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

A/D Conversion Techniques Using the AD7740
One method of using a VFC in an A/D system is to count the
output pulses of FOUT for a fixed gate interval (see Figure 9).
This fixed gate interval should be generated by dividing down
the clock input frequency. This ensures that any errors due to
clock jitter or clock frequency drift are eliminated. The ratio of
the FOUT frequency to the clock frequency is what is important
here, not the absolute value of FOUT. The frequency divi-
sion can be done by a binary counter where CLKIN is the
counter input.
Figure 10 shows the waveforms of CLKIN, FOUT, and the
Gate signal. A counter counts the rising edges of FOUT while the
Gate signal is high. Since the gate interval is not synchronized with
FOUT, there is a possibility of a counting inaccuracy. Depending
on FOUT, an error of one count may occur.
The clock frequency and the gate time determine the resolution
of such an ADC. If 12-bit resolution is required and CLKIN is
1 MHz (therefore, FOUT
time required is calculated as follows:
N counts at Full Scale (0.9 MHz) will take
N is the total number of codes for a given resolution; 4096 for
12 bits.
(N/0.9 × 10
minimum gate time = (4096/0.9 × 10
CLKIN
VIN
GATE
FOUT
GENERATOR
6
AD7740
) seconds = minimum gate time
CLOCK
CLKIN
FOUT
t
GATE
MAX
FREQUENCY
is 0.9 MHz), the minimum gate
DIVIDER
COUNTER
6
) seconds = 4.551 ms
GATE
SIGNAL
TO P
Since T
fastest conversion for a given resolution can be performed with
the highest CLKIN frequency.
If the output frequency is measured by counting pulses gated to
a signal derived from the clock, the clock stability is unimportant
and the device simply performs as a voltage-controlled frequency
divider, producing a high-resolution ADC. The inherent mono-
tonicity of the transfer function and wide range of input clock
frequencies allows the conversion time and resolution to be
optimized for specific applications.
Another parameter is taken into account when choosing the
length of the gate interval. Because the integration period of the
VFC is equal to the gate interval, any interfering signal can be
rejected by counting for an integer number of periods of the
interfering signal. For example, a gate interval of 100 ms will
give normal-mode rejection of 50 Hz and 60 Hz signals.
Isolation Applications
The AD7740 can also be used in isolated analog signal trans-
mission applications. Due to noise, safety requirements or distance,
it may be necessary to isolate the AD7740 from any controlling
circuitry. This can easily be achieved by using opto-isolators.
This is extremely useful in overcoming ground loops between
equipment.
The analog voltage to be transmitted is converted to a pulse
train using the VFC. An opto-isolator circuit is used to couple
this pulse train across an isolation barrier using light as the
connecting medium. The input LED of the isolator is driven
from the output of the AD7740. At the receiver side, the output
transistor is operated in the photo-transistor mode. The pulse
train can be reconverted to an analog voltage using a frequency-
to-voltage converter; alternatively, the pulse train can be fed into
a counter to generate a digital signal.
The analog and digital sections of the AD7740 have been designed
to allow operation from a single-ended power source, simplify-
ing its use with isolated power supplies.
Figure 11 shows a general purpose VFC circuit using a low cost
opto-isolator. A 5 V power supply is assumed for both the iso-
lated (V
VIN
V
DD
GATE
DD
AD7740
GND1
) and local (V
× FOUT
0.1 F
FOUT
MAX
10 F
CC
= number of counts at full scale, the
) supplies.
R
ISOLATION
BARRIER
OPTOCOUPLER
V
CC
AD7740
GND2