Analog Devices AD537SH, AD537SD-883B, AD537SD, AD537KH, AD537KD Datasheet

a		Integrated Circuit
		Voltage-to-Frequency Converter
		AD537*

FEATURES

Low Cost A–D Conversion

Versatile Input Amplifier

Positive or Negative Voltage Modes Negative Current Mode

High Input Impedance, Low Drift Single Supply, 5 V to 36 V Linearity: 0.05% FS

Low Power: 1.2 mA Quiescent Current Full-Scale Frequency up to 100 kHz 1.00 V Reference

Thermometer Output (1 mV/K) F-V Applications

MIL-STD-883 Compliant Versions Available

PIN CONFIGURATIONS

		D-14 Package						H-10A Package
									LOGIC GND
			AD537						10
							–VIN	1		9 OUTPUT
LOGIC GND	1			14	OUTPUT			AD537
SYNC	2		DRIVER	13	+VS				DRIVER
						+VIN	2			8	+VS
IIN
	3			12	CAP				CURR-
			CURR-						TO-FREQ
–VIN	4		BUF TO-FREQ	11	CAP			BUF	CONV
+VIN			CONV				3			7
	5			10	VOS			VT	PRECISION
VTEMP						VTEMP					CAP
	6	VT	PRECISION	9	V				VOLTAGE
					OS			VR REFERENCE
VREF	7	VR	VOLTAGE	8	–VS			4		6
			REFERENCE
							VREF		5	CAP
									–VS

(CONNECTED TO CASE)

PRODUCT DESCRIPTION

The AD537 is a monolithic V-F converter consisting of an input amplifier, a precision oscillator system, an accurate internal reference generator and a high current output stage. Only a single external RC network is required to set up any full-scale (F.S.) frequency up to 100 kHz and any F.S. input voltage up to

± 30 V. Linearity error is as low as ±0.05% for 10 kHz F.S., and operation is guaranteed over an 80 dB dynamic range. The overall temperature coefficient (excluding the effects of external components) is typically ±30 ppm/°C. The AD537 operates from a single supply of 5 V to 36 V and consumes only 1.2 mA quiescent current.

A temperature-proportional output, scaled to 1.00 mV/K, enables the circuit to be used as a reliable temperature-to- frequency converter; in combination with the fixed reference output of 1.00 V, offset scales such as 0°C or 0°F can be generated.

The low drift (1 µV/°C typ) input amplifier allows operation directly from small signals (e.g., thermocouples or strain gages) while offering a high (250 MΩ) input resistance. Unlike most V–F converters, the AD537 provides a square-wave output, and can drive up to 12 TTL loads, LEDs, very long cables, etc.

The excellent temperature characteristics and long-term stability of the AD537 are guaranteed by the primary bandgap reference generator and the low T.C. silicon chromium thin film resistors used throughout.

The device is available in either a 14-lead ceramic DIP or a 10-lead metal can; both are hermetically sealed packages.

*Protected by Patent Nos. 3,887,963 and RE 30,586.

REV. C

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

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 extended temperature range, –55°C to +125°C.

PRODUCT HIGHLIGHTS

1.The AD537 is a complete V-F converter requiring only an external RC timing network to set the desired full-scale frequency and a selectable pull-up resistor for the open collector output stage. Any full-scale input voltage range from

100 mV to 10 volts (or greater, depending on +VS) can be accommodated by proper selection of timing resistor. The full-scale frequency is then set by the timing capacitor from the simple relationship, f = V/10RC.

2.The power supply requirements are minimal, only 1.2 mA quiescent current is drawn from a single positive supply from 4.5 volts to 36 volts. In this mode, positive inputs can vary

from 0 volts (ground) to (+VS – 4) volts. Negative inputs can easily be connected for below ground operation.

3.F-V converters with excellent characteristic are also easy to build by connecting the AD537 in a phase-locked loop. Application particulars are shown in Figure 6.

4.The versatile open-collector NPN output stage can sink up to 20 mA with a saturation voltage less than 0.4 volts. The Logic Common terminal can be connected to any level be-

tween ground (or –VS) and 4 volts below +VS. This allows easy direct interface to any logic family with either positive or negative logic levels.

5.The AD537 is available in versions compliant with MIL- STD-883. Refer to the Analog Devices Military Product Databook or current AD537/883B data sheet for detailed specifications.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: [phone]	World Wide Web Site: http://www.analog.com
Fax: [phone]	© Analog Devices, Inc., 2000

AD537–SPECIFICATIONS (typical @ +25 C with VS (total) = 5 V to 36 V, unless otherwise noted)

				AD537KD	AD537SD1
Model		AD537JH	AD537JD	AD537KH	AD537SH1
CURRENT-TO-FREQUENCY CONVERTER
	Frequency Range	0 kHz to 150 kHz	*	*	*
	Nonlinearity1
	fMAX = 10 kHz	0.15% max (0.1% typ)	*	0.07% max	**
	fMAX = 100 kHz	0.25% max (0.15% typ)	*	0.1% max	**
	Full-Scale Calibration Error	± 10% max	± 7% max	± 5% max
	C = 0.01 µF, IIN = 1.000 mA				* *
	vs. Supply (fMAX < 100 kHz)	± 0.1%/V max (0.01% typ)	*	*	*
	vs. Temp (TMIN to TMAX)	± 150 ppm/°C max (50 ppm typ)	*	50 ppm/°C max (30 ppm typ)2	250 ppm/°C max
ANALOG INPUT AMPLIFIER
	(Voltage-to-Current Converter)
	Voltage Input Range
	Single Supply	0 to (+VS – 4) Volts (min)	*	*	*
	Dual Supply	–VS to (+VS – 4) Volts (min)	*	*	*
	Input Bias Current
	(Either Input)	100 nA	*	*	*
	Input Resistance (Noninverting)	250 MΩ	*	*	*
	Input Offset Voltage
	(Trimmable in “D” Package Only)	5 mV max	*	2 mV max	**
	vs. Supply	200 µV/V max	100 µV/V max	100 µV/V max	**
	vs. Temp (TMIN to TMAX)	5 µV/°C	*	1 µV/°C	10 µV/°C max
	Safe Input Voltage3	± VS	*	*	*
REFERENCE OUTPUTS
	Voltage Reference	1.00 Volt ± 5% max
	Absolute Value		*	*	*
	vs. Temp (TMIN to TMAX)	50 ppm/°C	*	100 ppm/°C max	**
	vs. Supply	± 0.03%/V max	*	*	*
	Output Resistance4	380 Ω	*	*	*
	Absolute Temperature Reference5
	Nominal Output Level	1.00 mV/K	*	*	*
	Initial Calibration @ +25°C	298 mV (± 5 mV typ)	*	298 mV (±5 mV max)	**
	Slope Error from 1.00 mV/K	± 0.02 mV/K	*	*	*
	Slope Nonlinearity	± 0.1 K	*	*	*
	Output Resistance5	900 Ω	*	*	*
OUTPUT INTERFACE (Open Collector Output)
	(Symmetrical Square Wave)
	Output Sink Current in Logic “0”
	VOUT = 0.4 V max (TMIN to TMAX)	20 mA min	20 mA min	20 mA min	10 mA min
	Output Leakage Current in Logic “1”				2 µA max
	(TMIN to TMAX)	200 nA max	*	*
	Logic Common Level Range	–VS to (+VS – 4) Volts	*	*	*
	Rise/Fall Times (CT = 0.01 µF)	0.2 µs
	IIN = l mA		*	*	*
	IIN = 1 µA	1 µs	*	*	*
POWER SUPPLY
	Voltage, Rated Performance
	Single Supply	4.5 V to 36 V	*	*	*
	Dual Supply	± 5 V to ±18 V	*	*	*
	Quiescent Current	1.2 mA (2.5 mA max)	*	*	*
TEMPERATURE RANGE		0°C to +70°C			–55°C to +125°C
	Rated Performance		*	*
	Storage	–65°C to +150°C	*	*	*
	PACKAGE OPTIONS6, 7
	D-14 Ceramic DIP		AD537JD	AD537KD	AD537SD
	H-10A Header	AD537JH		AD537KH	AD537SH

NOTES

*Specifications same as AD537JH. **Specifications same as AD537K.

1Nonlinearity is specified for a current input level (IIN) to the converter from 0.1 A to 1000 A. Converter has 100% overrange capability up to IIN = 2000 A with slightly reduced linearity. Nonlinearity is defined as deviation from a straight line from zero to full scale, expressed as a percentage of full scale.

2Guaranteed not tested.

3Maximum voltage input level is equal to the supply on either input terminal. However, large negative voltage levels can be applied to the negative terminal if the input is scaled to a nominal 1 mA full scale through an appropriate value resistor (See Figure 2).

4Loading the 1.0 volt or 1 mV/K outputs can cause a significant change in overall circuit performance, as indicated in the applications section. To maintain normal operation, these outputs should be operated into the external buffer or an external amplifier.

5Temperature reference output performance is specified from 0°C to +70°C for “J” and “K” devices, –55°C to +125°C for “S” model.

6D = Ceramic DIP; H = Hermetic Metal Can. For outline information see Package Information section.

7For AD537/883B specifications, refer to Analog Devices Military Products Databook.

Specifications subject to change without notice.

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REV. C

Applying the AD537

CIRCUIT OPERATION

Block diagrams of the AD537 are shown above. A versatile operational amplifier (BUF) serves as the input stage; its purpose is to convert and scale the input voltage signal to a drive current in the NPN follower. Optimum performance is achieved when, at the full-scale input voltage, a 1 mA drive current is delivered to the current-to-frequency converter. The drive current to the current-to-frequency converter (an astable multivibrator) provides both the bias levels and the charging current to the externally connected timing capacitor. This “adaptive” bias scheme allows the oscillator to provide low nonlinearity over the entire current input range of 0.1 µA to

2000 µA. The square wave oscillator output goes to the output driver which provides a floating base drive to the NPN power transistor. This floating drive allows the logic interface to be referenced to a different level 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 off the oscillator.

The reference generator uses a bandgap circuit (this allows single-supply operation to 4.5 volts which is not possible with low T.C. Zeners) to provide the reference and bias levels for the amplifier and oscillator stages. The reference generator also pro-

vides the precision, low T.C. 1.00 volt output and the VTEMP output which tracks absolute temperature at 1 mV/K.

V-F CONNECTION FOR POSITIVE INPUT VOLTAGES

The positive voltage input range is from –VS (ground in single supply operation) to 4 volts below the positive supply. The connection shown in Figure 1 provides a very high (250 MΩ) input impedance. The input voltage is converted to the proper drive current at Pin 3 by selecting a scaling resistor. The full-scale current is 1 mA, so, for example a 10 volt range would require a nominal 10 kΩ resistor. The trim range required will depend on capacitor tolerance. Full-scale currents other than 1 mA can be chosen, but linearity will be reduced; 2 mA is the maximum allowable drive.

As indicated by the scaling relationship in Figure 1, a 0.01 µF timing capacitor will give a 10 kHz full-scale frequency, and 0.001 µF will give 100 kHz with a 1 mA drive current. The maximum frequency is 150 kHz. Polystyrene or NPO ceramic capacitors are preferred for T.C. and dielectric absorption; polycarbonate or mica are acceptable; other types will degrade linearity. The capacitor should be wired very close to the AD537.

		1
GUARD RING		2
		3
R2	R1	4
OPTIONAL		5
INPUT VIN 10kΩ	10µF
FILTER		6
		7

		FO =	VIN
			10 (R1	+ R2) C
	AD537
		14	ROUT	fOUT
				+VS
	DRIVER	13
		12
	CURR-		C
	BUF TO-FREQ	11
	CONV
		10
VT	PRECISION	9	RT
			20k
	VOLTAGE
VR		8
	REFERENCE

Figure 1. Standard V-F Connection for Positive Input Voltages

V-F CONNECTIONS FOR NEGATIVE INPUT VOLTAGE OR CURRENT

A wide range of negative input voltages can be accommodated with proper selection of the scaling resistor, as indicated in Figure 2. This connection, unlike the buffered positive connection, is not high impedance since the 1 mA F.S. drive current must be supplied by the signal source. However, very large negative voltages beyond the supply can be handled easily; just modify the scaling resistors appropriately. Diode CR1 (HP50822811) is necessary for overload and latchup protection for current or voltage inputs.

If the input signal is a true current source, R1 and R2 are not used. Full-scale calibration can be accomplished by connecting a 200 kΩ pot in series with a fixed 27 kΩ from Pin 7 to –VS (see calibration section, below).

			IIN				FO =	VIN
FOUT		=						10 (R1	+ R2) C
			10C		AD537
			1			14			fOUT
0 TO –1mA								5kΩ (TYP)
			2		DRIVER	13			+VS
IIN
			3		CURR-	12
R1	CR1							C
					TO-FREQ	11
			4
R2					CONV
			5			10
			6	VT	PRECISION	9		20kΩ
VIN				VR	VOLTAGE
			7		REFERENCE	8
0 TO –10V

Figure 2. V-F Connections for Negative Input Voltage or Current

CALIBRATION

There are two independent adjustments: scale and offset. The first is trimmed by adjustment of the scaling resistor R and the second by the (optional) potentiometer connected to +VS and the VOS pins (“D” package only). Precise calibration requires the use of an accurate voltage standard set to the desired FS value and a frequency meter; a scope is useful for monitoring output waveshape. Verification of linearity requires the availability of a switchable voltage source (or a DAC) having a linearity error below ± 0.005%, and the use of long measurement intervals to minimize count uncertainties. Every AD537 is automatically tested for linearity, and it will not usually be necessary to perform this verification, which is both tedious and time-consuming.

Although drifts are small it is good practice to allow the operating environment to attain stable temperature and to ensure that the supply, source and load conditions are proper. Begin by setting the input voltage to 1/10,000 of full scale. Adjust the offset pot until the output frequency is 1/10,000 of full scale (for example 1 Hz for FS of 10 kHz). This is most easily accomplished using a frequency meter connected to the output. Then apply the FS input voltage and adjust the gain pot until the desired FS frequency is indicated. In applications where the FS input is small, this adjustment will very slightly affect the offset voltage, due to the input bias current of the buffer amplifier. A change of l kΩ in R will affect the input by approximately 100 µV, which is as much as 0.1% of a 100 mV FS range. Therefore, it may be necessary to repeat the offset and scale adjustments for the highest accuracy. The design of the input amplifier is such that the input voltage drift after offset nulling is typically below l µV/°C.

REV. C

–3–