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AD694AQ Datasheet(PDF) 9 Page - Analog Devices |
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AD694AQ Datasheet(HTML) 9 Page - Analog Devices |
9 / 16 page AD694 REV. B –9– Figure 8. Span Adjustment, 2 V Full Scale PROGRAMMING OTHER SPANS There are two methods for programming input spans less than 10 V. The first decreases the input span by programming a non- inverting gain into the buffer amplifier. For example, to achieve an input span of 0–5 V, the AD694 is set in its 10 V full-scale mode and the buffer amplifier is configured with a noninverting gain of 2 by adding 2 resistors. Now a 5 V signal at +Sig results in a 10 V full-scale signal at FB (Pin 1), the input to the V/I. This method requires that the V/I be programmed to a 10 V full scale for input spans between 2 V to 10 V. It should be pro- grammed to a 2 V full scale if input spans of less than 2 V are required. This adjustment scheme makes the accuracy of the span adjustment dependent upon the ratio accuracy of the re- quired gain resistors. Thus, it is possible to accurately configure spans other than 2 V or 10 V without using trimming potenti- ometers, given that the resistor ratios are sufficiently accurate. A supply voltage of 12.5 V is required for spans between 2 V and 10 V. Spans below 2 V require a VS of 4.5 V or greater. A second method, allows other spans of less than 10 V to be programmed when supply voltage is less than 12.5 V. Since the AD694 amplifiers require 2.5 V of headroom for operation, a 5 V full-scale input is possible with a 7.5 V supply. This is achieved by placing a resistor, in parallel with R2, (2 V FS [Pin 4] to Com [Pin 5]), to adjust the transconductance of the V/I converter without a headroom penalty. A disadvantage of this method is that the external resistor must match the internal re- sistor in a precise manner, thus a span trim will be required. The value should be chosen to allow for the ±10% uncertainty in the absolute value of the internal resistor R2. ADJUSTING REFERENCE OUTPUT Figure 9 shows one method of making small adjustments to the 10 V reference output. This circuit allows a linear adjustment range of ±200 mV. The 2 V reference may also be adjusted but only in the positive direction. Other reference voltages can be programmed by adding external resistors. For example, a resistor placed in parallel with R5 can be added to boost the reference output as high as 20 V. Con- versely, a resistor in parallel with R6 can be used to set the refer- ence voltage to a value between 2 V and 10 V. The output voltage VREF = 2 V (R6 + R5)/R5. In choosing external adjustment resistors remember that the internal resistors, while ratio matched to a high degree of accuracy, have an absolute re- sistor tolerance of only ±10%. Be prepared to compensate for this if a precise voltage other than the precalibrated values of 2 V or 10 V is required. Figure 9. 10 V Reference Output Adjustment BANDWIDTH CONTROL The bandwidth of the AD694 can be limited to provide noise filtering. This is achieved by connecting an external capacitor from BW ADJ (Pin 14) to VS (Pin 13) as shown in Figure 10. To program the bandwidth, substitute the desired bandwidth in Hz, into the formula below to determine the required capacitor. C BW =× × 12 900 /( ) πΩ The bandwidth chosen will vary ±10% due to internal resistor tolerance, plus an additional amount due to capacitor tolerance. This method of bandwidth control is not recommended as a way to filter large high frequency transients in the input signal. It is recommended that frequencies greater than the BW of the buffer amplifier be eliminated with an input filter to avoid recti- fication of noise by the input amplifiers. Figure 10. Noise Filtering with an External Capacitor BUFFER AMPLIFIER OFFSET ADJUST The buffer amplifier input voltage offset has been laser trimmed to a high degree of accuracy; however, there may be occasions when an offset trim is desired. Figure 11 shows the adjustment method; a trim range of greater than ±2.5 mV is available with this scheme. It is not recommended that this adjustment method be used to affect the 4 mA offset current as the trim will induce offset drift into the buffer amplifier. The buffer amplifier will drift approximately 1 µV/°C for each 300 µV of induced offset. To adjust the 4 mA offset current refer to the Adjusting 4 mA Zero section. |
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