AD537 is a new type of high precision, high linearity voltage/frequency converter introduced by American ADI (ANALOG DEVICES INC) company. It is composed of a high-impedance input amplifier, a precise oscillation system, an accurate internal reference generator and a high output current drive, which can directly receive small signals such as positive and negative voltages and currents; after simple transformation, it can also be used as a frequency / voltage converter, forming a phase-locked loop circuit.

AD537 can perform temperature compensation on the output with an accuracy of 1.00mV/K. At the same time, it can also be used as a reliable temperature/frequency converter, and can also be combined with a 1.00V reference voltage to compensate for the use of non-thermodynamic temperature Offset due to units such as Celsius, Fahrenheit, etc.

The AD537 has two packages: one is a 14-pin dual in-line type, and the other is a 10-pin metal can type.

The AD537 is available in three temperature and characterization grades, J, K, and S grades. Among them, the J and K grades are used in the range of 0°C to 70°C, while the S grades are used in the temperature range of -55°C to 125 °C.

The main features of AD537 are:

1. It is a complete voltage/frequency converter. It only needs to connect a resistor and a capacitor (to set the required full-scale frequency range) and a pull-up resistor when the collector is open to achieve voltage/frequency conversion. convert. The maximum full-scale input voltage range is 30V , and the corresponding full-scale output frequency range is 100kHz. The relationship between the full-scale output frequency range and the external resistors and capacitors is: f=V/(10×RC).

2. Its linearity is very high. When the full-scale output frequency range is 10kHz, its nonlinearity can reach 0.05%, and the dynamic range of the input voltage can be guaranteed to exceed 80dB.

3. The required power is very low, only 1.2mA quiescent current is needed when the power supply voltage is 4.5-3.6V in unipolar operation.

4. If the AD537 is connected to a phase-locked loop, a good frequency/voltage conversion can be achieved.

5. Strong driving ability. When the saturation voltage drop is less than 0.4V, the open-collector output stage can sink up to 20mA and drive 12 TTL loads.

6. It is less affected by temperature, and the overall temperature coefficient (including the influence of peripheral devices) can usually reach 30ppm/℃.

7. Compatible with military standard MIL-STD-883.

feature

Low cost AD conversion

Multifunction Input Amplifier

Positive or Negative Voltage Mode

Negative current mode

High input impedance, low drift

Single supply, 5 V to 36 V

Linearity: 0.05% FS

Low power consumption: 1.2 mA quiescent current

Full-scale frequency up to 100 kHz

1.00 V reference

Thermometer output (1 mV/K)

FV app

MIL-STD-883 Compliant Version

PIN configuration diagram

Product Description

The AD537 is a monolithic VF converter amplifier consisting of an input, a precision oscillator system, an accurate internal reference generator and a high current output stage. Only one setting any full scale (FS) requires an external RC network frequency up to 100 kHz and any FS input voltage up to ±30 V. For 10 kHz FS, linearity errors as low as ±0.05% are guaranteed to operate over an 80 dB dynamic range. Overall temperature coefficient (excluding external effects components) is typically ±30ppm/°C. The AD537 operates from a single supply of 5 V to 36 V and consumes only 1.2 mA of quiescent current. A temperature proportional output, with a scale of 1.00 mV/K, enables the circuit to be used as a reliable temperature-to-frequency converter; combined with a fixed reference, the output is 1.00 V, which can generate offset scales such as 0°C or 0°F. The low drift (1µV/°C typical) input amplifier allows operation directly from small signals (eg, thermocouples or strain gauges) while providing high (250MΩ) input resistance. unlike most people

The AD537 provides a VF converter that provides a square wave output that can drive up to 12 TTL loads, LEDs, very long cables, etc.

The excellent temperature characteristics and long-term stability of the main bandgap reference guarantee the performance of the AD537 generator and low TC silicon chromium thin film resistors throughout use. The device is available in a 14-pin ceramic DIP or a 10-pin metal can; both are hermetically packaged. The AD537 is available in three performance/temperature grades; the J and K grades are specified for operation over the 0°C to +70°C range, while the AD537S is specified for operation over the -55°C to +125°C extended temperature range.

Product Highlights

1. The AD537 is a complete VF converter requiring only an external RC timing network to set the desired full-scale frequency and an optional pull-up resistor for the open collector output stage. Any full-scale input voltage range of 100 mV to 10 volts (or higher, depending on +VS) can be accommodated with proper selection of timing resistors. The full-scale frequency is then set by a timing capacitor with a simple relationship, f = V/10RC.

2. Extremely low power requirements, only 1.2 mA quiescent current from a single positive supply

4.5 volts to 36 volts. In this mode, the positive input can vary from 0 volts (ground) to (+ VS - 4) volts. The negative input can be easily connected for underground operation.

3. An FV converter with excellent characteristics is also easy to build by connecting AD537 in a phase locked loop.

4. The multifunctional open-collector NPN output stage can sink to 20 mA when the saturation voltage is less than 0.4 volts. The Logic Common terminal can be connected to any level ground (or -VS) and 4 volts below +VS. This allows for a simple and direct interface to any logic family, either positive or negative logic levels.

circuit operation

The block diagram of the AD537 is shown above. The Versatile Operational Amplifier (BUF) acts as the input stage; its purpose is to convert and scale the input voltage signal to the current condition of the driver NPN fan. The best performance is achieved when the full-scale input voltage, dynamic current is 1 mA delivered to the current-to-frequency converter. A drive current-to-current-to-frequency converter (a very stable multivibrator) provides bias levels and charging current to externally connected timing capacitors. This "adaptive" biasing scheme allows the oscillator to provide low nonlinearity over the entire current input range of 0.1µA to 2000µA. Square wave oscillator output goes to output

Drivers that provide floating base driver transistors for NPN power supplies. This floating driver allows references to logical interfaces at different levels than -VS. The "SYNC" input ("D", package only) allows the oscillator to be slaved to an external master oscillator; this input can also be used to shut down the oscillator. The reference generator uses a bandgap circuit (this allows single-supply operation at 4.5 volts, which is not possible with low TC zeners) to provide reference and bias levels for the amplifier and oscillator stages. Reference generators are also available

Low precision, TC. 1.00 volt output and VTEMP output track absolute temperature to 1 mV/K.

VF connection for positive input voltage The positive voltage input range is -VS (single ground, powered operation) 4 volts below the positive supply. The connections are shown below, providing a very high (250MΩ) input impedance. input voltage is converted to the correct driver

By choosing a scaling resistor, a current is generated on pin 3. The full current is 1 mA, so for example a 10 volt range would require a nominal 10kΩ resistor. The required adjustment range depends on the capacitance tolerance. Full-scale currents other than 1 mA can be used

choice, but linearity will be reduced; 2 mA is the maximum allowed for the drive. As shown in the scaling relationship in Figure 1, it is 0.01 μF

The timing capacitor will provide 10 kHz full-scale frequency, and 0.001µF will provide 1 mA drive current at 100 kHz. The maximum frequency is 150 kHz. Polystyrene or NPO ceramic capacitors are the first choice for TC. and dielectric absorption; polycarbonate or mica are acceptable; other types will degrade linearity. The capacitor should be very close to the wiring AD537.

Standard VF connection voltage for positive input

VF connection for negative input voltage or current

Can adapt to various negative input voltages

Correctly select the scaling resistor as shown

2. This connection is not the same as the buffered positive connection, since the 1 mA FS impedance is not high. The drive current must be provided by the signal source. However, negative voltages very large beyond the supply can be easily handled; just modify the appropriately scaled resistors. Diode CR1 (HP50822811) is the voltage input necessary for current or current overload and latch-up protection. If the input signal is a real current source, then R1 and R2 are not used. Full calibration can be done by connecting a series 200kΩ potentiometer, pin 7 to -VS fixed 27kΩ (see Calibration section, see below).

Negative input voltage or the VF connection of the input voltage current

calibration

There are two independent adjustments: scale and offset. The first is to adjust a second (optional) potentiometer by adjusting the scaling resistor R and the trim resistor connected to the +VS and VOS pins ("D" package only). Accurate calibration requires the use of an accurate voltage standard set to the desired FS value and a frequency meter; ranges are useful for monitoring output waveforms. Linear verification requires the availability of a

Switchable voltage sources (or DACs) with linearity errors are below ±0.005% and use long measurement intervals to minimize counting uncertainty. Every AD537 is automatically tested for linearity, and it is usually not necessary to perform this verification, which is tedious and time-consuming. While drift is small, it is a good practice to allow operation in an environment to achieve stable temperatures and ensure supply, source and load conditions are appropriate. Input voltage is 1/10,000 of full scale from setup. Adjust the offset until the output frequency is 1/10,000 of full scale (e.g. 1 Hz when FS is 10 kHz). This is easily accomplished using a frequency meter connected to the output. Then apply the FS input voltage and adjust the gain potential until the desired FS indicated frequency is reached. In applications where the FS input is small, this adjustment will have a slight effect on the offset voltage, due to the input bias current of the buffer amplifier. Changing the lkΩ in will affect the input by about 100µV, or up to 0.1% of the 100 mV FS range. Therefore, it may be necessary to repeat the offset and scale adjustment for maximum accuracy. The input amplifier is designed like this

Input voltage drift after offset zeroing is typically less than 1 μV/°C.

In some cases, the signal can be a resource in the form of a negative current. This handles the input voltage in a similar way to negation. However, scaling resistors are no longer required, eliminating the ability to trim full scale in this fashion. Since changing capacitance is often impractical, another calibration scheme is required. This is shown in the image below. A resistor potentiometer is connected

The VR output to -VS will change the internal operating conditions in a predictable manner, providing the necessary adjustment range. Using the values shown, a range of ±4% can be obtained; larger ranges can be achieved by reducing R1. This technique does not reduce the temperature coefficient of the converter, and the linearity is the same as the negative input voltage. Minimum supply voltage can be used. There is no need to adjust the offset unless the input node needs to be set at the exact ground potential. Choose capacitor C. 5% below nominal; where R2 has a mid-position output frequency given by:

f = I

10.5 × C

where f is in kHz, I is mA, and C is μF. For example, with an aFS input of 1 mA, and a FS frequency of 10 kHz, C = 9500 pF. Calibrating R2 for the correct reading is achieved by applying the full scale input and adjustment. This alternative adjustment scheme can also be used when it is desired to present an accurate input resistance pattern in negative voltages. Then there is the scaling relationship

f = V.

REXACT

×

1

10.5C

The calibration procedure is then similar to that used for the positive input voltage, except that R2 is adjusted by squaring.

Scaling of current input

input protection

The AD537 is designed to use minimal hardware time. However, the precise successful application of the IC can provide a good understanding of possible pitfalls using appropriate precautions. The -VIN, +VIN and IIN pins should not be driven more than 300 mV below -VS. This causes internal connections to be made, possibly destroying the IC. The AD537 can be protected from a Schottky diode CR1 (HP5082-2811) from the "below -VS" input as shown in Figure 3. It is also desirable not to drive +VIN, -VIN and IIN above +VS. In operation, the converter will become very nonlinear for inputs above (+VS - 3.5 V). Controlling currents higher than 2 mA can also cause nonlinearity.

The AD537's 80 dB dynamic range ensures operation from a control current of 1 mA (nominal FS) down to 100 nA (equivalent to 1 mV to 10 V FS). Improperly operated oscillators below 100 nA may result in false indications of input amplitude. In many cases this may be due to brief noise spikes being added to the input. For example, when the zoom accepts an FS input of 1 V, the -80 dB level is only 100 μV, so a noise spike of 0.9 mV is enough to cause a momentary noise glitch when the average input is only 60 dB below FS (1 mV).

This effect can be minimized by using a simple low-pass filter before the converter and a guard ring around IIN or -VIN

pin. For a FS of 10 kHz, a single-pole filter with a time constant of 100 ms (Figure 2) would be suitable, but the optimal configuration will depend on the application and type of signal processing. Noise spikes are only possible causes of errors where the input current remains near its minimum value for a prolonged period of time above 100 nA (1 mV) of additive input fully integrated noise occurs.

The AD537 is slightly susceptible to other interfering signals. The most sensitive nodes (besides the input) are the capacitor terminals and the SYNC pin. Timing capacitors should be placed as close to the AD537 as possible to minimize signal pickup in the leads. In some cases, guard rings or shielding may be necessary. The SYNC pin should be decoupled via a 0.005µF (or larger) capacitor to pin 13 (+VS). This minimizes the possibility of the AD537 trying to synchronize a false signal. This precaution is unnecessary metal cans can be packaged because there is no SYNC function output to the package pins and therefore not easy to pick up. Decoupling It is good engineering practice to use bypass capacitors on the circuit board for supply voltage pins and insert small value resistors (10Ω to 100Ω in the supply line) to provide a measure of decoupling between the various circuits in the system. between. A ceramic capacitor should apply 0.1µF to 1.0µF of current between the supply voltage pins and the analog signal to ground in order to properly bypass the AD537. Decoupling capacitors can also be used from +VS to SYNC in those applications where very little cycle-to-cycle variation (jitter) is required. This noise is reduced by placing a capacitor SYNC across +VS and +. A 6.8µF capacitor reduces jitter to 20,000 in the 10 kHz FS range, which is sufficient for most applications. Tantalum capacitors should be used to avoid errors due to DC leakage.

The figure below shows the AD537's standard 0 to +10 V input connections and output stage connections. The values of logic common voltage, pull-up resistors, positive logic levels and -VS power supply are given in the attached diagram for several logic forms.

Connecting Standard Logic Series

application

The diagrams and instructions in the following application are provided to stimulate the discerning engineer with alternative circuit design ideas. "AD537 IC Voltage-to-Frequency Application Converter", available on request from Analog Devices, covers a wider range of topics and concepts in Data Using Voltage-to-Frequency Conversion and Data Transfer Converters. True two-wire data transmission

The figure below shows the AD537 scheme in true two-wire data transfer. Twisted pair transmission lines have the dual purpose of supplying power to devices and also carry data in the form of frequency current modulation. The PNP circuit at the receiver represents a fairly simple way of converting current modulation back into a voltage square wave that will directly drive the digital logic. Among them, the 0.6 volt square wave will appear on the supply line of the equipment terminal without affecting the performance of the AD537 due to its excellent performance. Supply rejection. Also, note that the circuit runs almost constant average power regardless of frequency.

true two-wire operation