AD7741BRZ Analog Devices Inc, AD7741BRZ Datasheet - Page 9

• Manufacturer Part Number: AD7741BRZ
• Description: IC CONVERTER V TO FREQ 8-SOIC
• Manufacturer: Analog Devices Inc
• Type: Voltage to Frequencyr

Specifications of AD7741BRZ

• Frequency - Max: 3.072MHz
• Full Scale: ±50ppm/°C
• Linearity: ±0.012%
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Frequency: 2.75MHz
• Full Scale Range: 3.072MHz
• Linearity %: 0.01%
• Supply Voltage Range: 4.75V To 5.25V
• Digital Ic Case Style: SOIC
• No. Of Pins: 8
• Msl: MSL 3 - 168 Hours
• Frequency Max: 6.144MHz
• Converter Function: VFC
• Full Scale Frequency: 1350
• Power Supply Requirement: Single
• Single Supply Voltage (typ): 5V
• Single Supply Voltage (max): 5.25V
• Single Supply Voltage (min): 4.75V
• Dual Supply Voltage (typ): Not RequiredV
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Package Type: SOIC N
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD7741/42EBZ - BOARD EVAL FOR AD7741/7742
• Lead Free Status / Rohs Status: Compliant

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
quency of 0.05 f
either standard logic families or opto-couplers. The clock high
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
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.
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
f
OUT
Figure 7. AD7741/AD7742 Frequency Output Waveforms
f
OUT
f
OUT
= f
V
V
V
IN
= f
IN
IN
CLKIN
= f
= V
= V
= V
CLKIN
CLKIN
f
CLKIN
REF
REF
REF
*3/20
/10
/4
/2
/8
/4
AVERAGE f
VARIES BETWEEN f
CLKIN
CLKIN
6 T
. The output allows simple interfacing to
and zero-scale input gives an output fre-
CLK
OUT
IS f
CLKIN
CLKIN
*3/20 BUT THE ACTUAL PULSE STREAM
/6 AND f
CLKIN
/7
7 T
CLK
–9–
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
bandgap reference. For the AD7741, this is done by simply
leaving REFIN/OUT unconnected. For the AD7742, REFIN is
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 W max.
Figure 8. On-Chip Oscillator
CLKIN
AD7741/AD7742
C1
5M
AD7741/AD7742
CLKOUT
C2
TO OTHER
CIRCUITRY