feature

±10 V tracking output; Kelvin connection; low tracking error: 1.5 mV; low initial error: 2.0 mV; low drift: 1.5 ppm/°C; low noise: 6 μV pp; flexible output force and sense termination; high impedance ground sense ; Wide body SOIC and CERDIP packages.

General Instructions

The AD688 is a high precision ± 10V tracking reference. Low Tracking Error, Low Initial Error, and Low Temperature Drift The AD688 Reference Absolute ±10V Accuracy Performance was previously unavailable in monolithic form. The AD688 uses patented ion implanted buried Zener diodes and laser wafers for excellent performance in drift trimming of high stability thin film resistors.

The AD688 consists of a basic reference unit and three additional amplifiers. The amplifier is laser trimmed for low offset and low drift and maintains reference accuracy. The configuration of the amplifier allows Kelvin connections to the load and/or booster to drive long lines or high current loads, providing the full precision of the AD688 when required in the application circuit.

The low initial error allows the AD688 to be used as a system reference for precision measurement applications requiring 12-bit absolute accuracy. In such a system, the AD688 can provide a known voltage for system calibration; thus eliminating the cost of periodic recalibration. In addition, the use of the AD688 and calibration software eliminates the mechanical instability of the trimmer potentiometer and the possibility of incorrect calibration.

There is a commercial version of the AD688. The AD688 is available in wide-body 16-lead SOIC and 16-lead CERDIP packages over the -40C to + 85C temperature range.

Product Highlights

1. Accurate tracking. The AD688 provides accurate tracking ±10 V Kelvin output connections without the need for external components. With tracking error of less than 1.5 mV, the finetrim can be used in applications requiring precise symmetry between the +10 V and -10 V outputs.

2. Accuracy. The AD688 provides 12-bit absolute accuracy without any user adjustment. Optional trim connections are available for applications requiring higher accuracy. Trimming does not change the operating conditions of the Zener or buffer amplifier and therefore does not increase temperature drift.

3. Low output noise. The output noise of the AD688 is low, typically 6µV pp. Use external capacitors to provide pins for broadband noise filtering.

Absolute Maximum Ratings

Stresses above those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. This is a stress rating only and does not imply that the functional operation of the device under these or any other conditions is higher than that shown in the operating section of this specification. Prolonged exposure to absolute maximum rating conditions may affect reliability.

theory of operation

The AD688 consists of a buried Zener diode reference, amplifier, and associated thin-film resistors, as shown in Figure 3. A temperature compensation circuit provides the device with a temperature coefficient of 1.5ppm/°C or less.

Amplifier A1 performs several functions. A1 is mainly used to amplify the Zener voltage to the desired 20 V. In addition, A1 provides external adjustment of the 20 V output via pin 5 (gain adjustment). Using a bias compensation resistor between the Zener output and the non-vertical input of A1, a capacitor can be added at the noise reduction pin (Pin 7) to form a low pass filter and reduce the Zener's noise contribution to the circuit. Two matched 12 kΩ nominal thin film resistors (R4 and R5) split the 20 V output in half.

The ground sense of the circuit is provided by amplifier A2. The non-vertical input (pin 9) senses system ground and forces the midpoint of resistors R4 and R5 to virtual ground. Pin 12 (BAL ADJ) can be used to fine tune this midpoint transmission.

Amplifiers A3 and A4 are internally compensated to buffer the voltage at pins 6 and 8 and provide a full Kelvin output. Therefore, the AD688 is fully Kelvin capable by providing a means to sense the system ground and forcing and sensing outputs referenced to that ground.

application

The AD688 can be configured to provide ±10 V reference outputs, as shown in Figure 4. The AD68's architecture provides ground sensing and an uncommitted output buffer amplifier, giving the user a great deal of functional flexibility. The AD688 is specified and tested in the configuration shown in Figure 4. The user may choose to take advantage of other configuration options provided by the AD688; however, performance in these configurations is not guaranteed to meet strict data sheet specifications.

Unbuffered outputs are available at pins 6 and 8. Loading these unbuffered outputs will hurt circuit performance.

Amplifiers A3 and A4 can be used interchangeably. However, the AD688 was tested (and spec'ed) with the amplifier connections shown in Figure 4. When A3 or A4 is not in use, its output force and sense pins should be connected, and the input should be grounded.

Two outputs of the same voltage polarity can be obtained by connecting A3 and A4 to the appropriate unbuffered output on pin 6 or pin 8. The performance of these dual output configurations is generally within data sheet specifications.

calibration

In general, the AD688 will meet the requirements of a precision system without additional tuning. The initial output voltage error is 2 mV, and the output noise specification is 6 μV pp, allowing an accuracy of 12 to 16 bits. However, in applications requiring higher accuracy, additional calibration may be required. Fine-tuning is done by using the Gain Adjust and Balance Adjust pins (Pin 5 and Pin 12 respectively).

The AD688 provides a 20V accurate span and a center tap that is used with buffers and ground sense amplifiers to achieve a ±10V output configuration. The GAIN ADJ and BAL ADJ can be used to fine-tune the magnitude of the 20 volt span voltage and the position of the center tap within the span. Gain adjustment should be done first. Although trimming is not interactive within the device, gain trimming moves the balance trimming point when changing the range size.

Figure 5 shows the gain and balance trim of the AD688. Each trim uses a 100 kΩ 20-turn potentiometer. The gain trim pot is connected between pin 6 (V) and pin 8 (V), and the wiper is connected to pin 5 high and low (gain adjustment). Adjust the potentiometer so that it produces 20 volts between pin 1 and pin 15 of the amplifier output. Then, adjust the balance potentiometer (also connected between pins 6 and 8, and connect the wiper to pin 12 (BAL ADJ)) to span from +10 V to -10 V.

The input impedance on the GAIN ADJ and BAL ADJ pins is approximately 150 KΩ. The gain adjustment trim network effectively attenuates the 20 volts on the trim pot by a factor of about 1150 to provide a trim range of about 5.8 mV to +12 mV with a resolution of about 900 V/turn (20-turn pot). The balance adjustment trim network attenuates the trim voltage by a factor of about 1250, providing a trim range of ±8 mV with a resolution of 800 µV/rev. Trimming the AD688 produces no additional temperature error, so precision potentiometers are not required. Pin 12 should remain floating when no balance adjustment is required. Pin 5 should also be left floating if no gain adjustment is required.

Noise performance and reduction

The noise generated by the AD688 is typically less than 6µV pp over the 0.1hz to 10hz frequency band. The noise in a 1 MHz bandwidth is about 840 mega VPP. The main source of this noise is the buried Zener, which contributes about 140 nV/z Hz. In contrast, the contribution of the op amp is negligible. Figure 6 shows the 0.1 Hz to 10 Hz noise of a typical AD688.

If further noise reduction is required, an optional capacitor can be added between the noise reduction pin and ground, as shown in Figure 5. This will form a low pass filter with 5 kΩ R on the output of the zener unit. The 1µF capacitor has a 3dB point at 32Hz and reduces high frequency noise (down to 1MHz) to about 250µV pp. Figure 7 shows the 1 MHz noise of a typical AD688 with or without a 1µF capacitor.

opening time

When power is applied (cold cranking), the time it takes for the output voltage to reach its final value within a specified tolerance is the turn-on settling time. The two components usually associated with this are: the time the active circuit settles and the time the thermal gradient on the chip settles. Figure 8 and Figure 9 show the turn-on characteristics of the AD688. They show a settling time of about 600 microseconds. Note that when the horizontal scale is extended to 2 ms/cm in Figure 9, there is not any thermal tail.

When using an external noise reduction capacitor, the output turn-on time is modified. When present, this capacitor provides additional load to the current source of the internal Zener diode, resulting in a slightly longer power-on time. In the case of a 1 μF capacitor, the initial turn-on time is about 100 ms (Figure 10).

When using noise reduction, a 20 kΩ resistor is required between pins 6 and 2 for proper startup.

temperature performance

The AD688 is designed for precision reference applications where temperature performance is critical. Extensive temperature testing ensures that the device's high performance levels remain unchanged over the operating temperature range.

Figure 11 shows a typical output voltage drift and illustrates the test method. The box in Figure 11 is bounded on both sides of the operating temperature extremes, at the top and bottom by the maximum and minimum +10 V output error voltages measured over the operating temperature range. The slopes of the diagonal lines drawn for the +10 V and -10 V outputs determine the performance level of the device.

Each AD68A and B grade unit is tested at 40°C, 25°C, 0°C, +25°C, +50°C, +70°C, and +85°C. This method ensures that the change in output voltage that occurs over a temperature change within the specified range will be contained within a box with a slope equal to the slope of the maximum specified drift. The position of the box on the vertical scale changes from device to device as the initial error and the shape of the curve change. In the appropriate temperature range, the maximum height of the box is shown in Figure 12.

Reproducing these results requires a combination of high accuracy and stable temperature control in the test system. Evaluation of the AD688 will yield a curve similar to that in Figure 11, but the output readings may vary depending on the test method and equipment used.

Kelvin connection

Force and sense connections, also known as Kelvin connections, provide a convenient way to eliminate the effects of voltage drops in circuit lines. As shown in Figure 13a, the load current and wire resistance produce errors (V=R×I) when loaded. The Kelvin connection in Figure 13b overcomes this problem by including wire resistance and inductive load voltage in the forced loop of the amplifier. The amplifier corrects for any errors in the load voltage. In the circuit shown, the output of the amplifier is actually 10 V+V and the load voltage is the desired 10 V.

The AD688 has three amplifiers that can be used to implement Kelvin connections. Amplifier A2 is dedicated to the ground force sensing function, while amplifiers A3 and A4 are not committed to do other force sensing work for free.

Unused amplifiers can also be used for other circuit functions. Figures 14 to 19 show the typical performance of A3 and A4. In some applications, one amplifier may not be used. In this case, the unused amplifier should be connected as a unity gain follower (the force and sense pins are connected together) and the input should be grounded.

Dynamic performance

The output buffer amplifiers (A3 and A4) are designed to provide static and dynamic load regulation to the AD688 over a less complete reference.

Many A/D and D/A converters impose transient current loads on the reference, and poor reference response can degrade the converter's performance.

Figure 20, Figure 21, and Figure 22 show the characteristics of the AD688 output amplifier driving a 0 mA to 10 mA load.

Figure 23 and Figure 24 show the output amplifier characteristics driving a 5 mA to 10 mA load, a common situation found when a reference is shared among multiple converters or used to provide bipolar bias currents.

In some applications, the variable load can be resistive and capacitive in nature, or can be connected to the AD688 through long capacitive cables. Figure 25 and Figure 26 show the output amplifier characteristics driving a 1000 pF, 0 mA to 10 mA load.

Figure 27 and Figure 28 show the crosstalk between the output amplifiers. The top trace shows the output of A4, DC coupled and offset by 10 volts, while the output of A3 is subjected to a load current step of 0mA to 10mA. The transient settling at A4 is about 1 μs, and the load-induced offset is about 100 μV.

Attempting to drive a large capacitive load (over 1000 pF) can cause ringing or oscillation, as shown in the step response photo (Figure 29). This is due to the additional pole formed by the load capacitance and amplifier output impedance, which consumes phase margin. Figure 30 shows the recommended method for driving capacitive loads of this magnitude. The 150Ω resistor isolates the capacitive load from the output stage, while the 10 kΩ resistor provides a DC feedback path and maintains output accuracy. A 1µF capacitor provides a high frequency feedback loop. The performance of this circuit is shown in Figure 31.

Bridge drive circuit

A Wheatstone bridge is a common sensor. In its simplest form, the bridge consists of four 2-terminal elements connected in a quadrilateral, the excitation source is connected along one of the diagonals, and the detector contains the other diagonal.

In this unipolar drive configuration, the output voltage of the bridge rides on a common-mode voltage signal equal to about V/2. Further processing of this signal must be limited to high common-mode rejection techniques such as instrumentation or isolation amplifiers. However, if the bridge is driven by a pair of bipolar supplies, the common-mode voltage is ideally eliminated and the constraints on any subsequent processing elements are relaxed.

As shown in Figure 32, the AD688 is the best choice for the control components in a bipolar bridge driver scheme. Transistors Q1 and Q2 act as series pass elements to increase the current drive capability to the 57mA required for a typical 350Ω bridge. If the bridge balance is not ideal, a differential gain stage may still be required.

Dimensions