AD7740YRM-REEL Analog Devices Inc, AD7740YRM-REEL Datasheet - Page 7

• Manufacturer Part Number: AD7740YRM-REEL
• Manufacturer: Analog Devices Inc

Specifications of AD7740YRM-REEL

• 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): -40C
• Operating Temperature (max): 105C
• Operating Temperature Classification: Industrial
• Package Type: MSOP
• Lead Free Status / Rohs Status: Not Compliant

GENERAL DESCRIPTION
The AD7740 is a CMOS synchronous Voltage-to-Frequency
Converter (VFC) which uses a charge-balance conversion
technique. The input voltage signal is applied to a proprietary
front-end based around an analog modulator which converts the
input voltage into an output pulse train.
The part also contains an on-chip 2.5 V bandgap reference and
operates from a single 3.3 V or 5 V supply. A block diagram of
the AD7740 is shown in Figure 3.
Input Amplifier Buffering and Voltage Range
The analog input VIN can be buffered by setting BUF = 1. This
presents a high impedance, typically 100 MΩ, which allows
significant external source impedances to be tolerated. The VIN
voltage range is now 0.1 V to VDD – 0.2 V. By setting BUF = 0
the AD7740 input circuit accepts an analog input below GND
and the analog input VIN has a voltage range from –0.15 V to
VDD + 0.15 V. In this case the input impedance is typically
650 kΩ.
The transfer function for the AD7740 is represented by:
It is shown in Figure 4 for unbuffered mode.
Sample Calculation:
V
FOUT (min) = 0.1 f
FOUT (max) = 0.1 f
FOUT MIN
REF
VIN
FOUT MAX
0.10 f
0.90 f
–0.15V
= 2.5 V, BUF = 0
CLKIN
CLKIN
FOUT = 0.1 f
BUF
0
OUTPUT
FREQUENCY
FOUT
AD7740
= 0.052 f
= 0.948 f
GND
CLKIN
CLKIN
SWITCHED
SWITCHED
CLKIN
CLKIN
CLKIN
CAPS
CAPS
+ 0.8(–0.15/2.5) f
+ 0.8(2.65/2.5) f
+ 0.8 (VIN/V
INTEGRATOR
V
REF
REF
V
CLKIN
REF
CLKIN
) f
AD7740
COMPARATOR
+ 0.15V
CLKIN
FOUT
INPUT
VOLTAGE
VIN
VFC Modulator
The analog input signal to the AD7740 is continuously sampled
by a switched capacitor modulator whose sampling rate is set
by a master clock. The input signal may be buffered on-chip
(BUF = 1) before being applied to the sampling capacitor of the
modulator. This isolates the sampling capacitor charging currents
from the analog input pin.
This system is a negative feedback loop that acts to keep the net
charge on the integrator capacitor at zero, by balancing charge
injected by the input voltage with charge injected by V
output of the comparator provides the digital input for the 1-bit
DAC, so that the system functions as a negative feedback loop
that acts to minimize the difference signal. See Figure 5.
The digital data that represents the analog input voltage is con-
tained in the duty cycle of the pulse train appearing at the output
of the comparator. The output is a pulse train whose frequency
depends on the analog input signal. A full-scale input gives an
output frequency of 0.9 f
output frequency of 0.1 f
facing to either standard logic families or opto-couplers. The
pulsewidth of FOUT is fixed and is determined by the high period
of CLKIN. The pulse is synchronized to the rising edge of the
clock signal. The delay time between the edge of CLKIN and the
edge of FOUT is typically 35 ns. Figure 6 shows the waveform
of this frequency output. (See TPC 8.)
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
two CLKIN cycles.
INPUT
FOUT = f
FOUT = f
FOUT = f
AVERAGE FOUT IS f
BETWEEN f
VIN = V
VIN = V
VIN = V
CLKIN
CLKIN
CLKIN
f
CLKIN
REF
REF
REF
3/10
CLKIN
/2
/2
/5
/8
/4
INTEGRATOR
/3 and f
3
t
–V
CLKIN
CLKIN
REF
CLKIN
CLKIN
CLKIN
3/10 BUT THE ACTUAL PULSE STREAM VARIES
/4
COMPARATOR
. The output allows simple inter-
+V
and zero-scale input gives an
REF
4t
CLKIN
CLK
AD7740
AD7740
1-BIT
BITSTREAM
REF
. The