Datasheet AD693 (Analog Devices) - 7

AD693
Figure 11. Using an External Pass Transistor to Minimize Self-Heating Errors of adjustment of the output current from nominal. Substitute 0-to-75 mV signal in the 0-to-20 mA mode). The gain of this this value in the appropriate formula below for adjustment at the amplifier is trimmed to 2.00 so that an input signal ranging from 4 mA tap. 0-to-30 mV will drive the V/I section to produce 4-to -20 mA. R Joining P1 and P2 (Pins 15 and 16) will reduce the Signal Ampli- Z1 = (1.6 V/IA) – 400 Ω and fier gain to one, thereby requiring a 60 mV signal to drive the V/I RZ2 = RZ1 × 3.1 V/(15 mV + IA × 3.75 Ω) to a full 20 mA span. Use a similar connection with the following resistances for To produce spans less than 30 mV, an external resistor, RS1, can adjustments at the 12 mA tap. be connected between P1 and 6.2 V. The nominal value is given R by: Z1 = (4.8 V/IA) – 400 Ω and RZ2 = RZ1 × 3.1 V/(45 mV + IA × 3.75 Ω) R = 400 Ω S1 30 mV These formulae take into account the ± 10% tolerance of tap −1 resistance and insure a minimum adjustment range of I S A. For example, choosing I where S is the desired span. For example, to change the span to A = 200 µA will give a zero adjustment range of ± 1% of the 20 mA full-scale output. At the 4 mA tap the 6 mV a value of: maximum value of: R = 400 Ω = 100 Ω S1 R 30 mV Z1 = 1.6 V/200 µA – 400 Ω = 7.6 kΩ and −1 6 mV R Z2 = 7.6 kΩ × 3.1 V/(15 mV + 200 µA × 3.75 Ω) = 1.49 MΩ is required. Since the internal, 800 Ω gain setting resistors exhibit an absolute tolerance of 10%, RS1 should be provided with up to ± 10% range of adjustment if the span must be well controlled. For spans between 30 mV and 60 mV a resistor RS2 should be connected between P1 and P2. The nominal value is given by:   400 Ω 1− 60 mV  S  R = S2 30 mV −1 S For example, to change the span to 40 mV, a value of:   400 Ω 1 − 60 mV  40 mV  R = = 800 Ω S2 30 mV −1 40 mV Figure 12. Optional 4 mA Zero Adjustment (12 mA Trim Available Also) is required. Remember that this is a nominal value and may require adjustment up to ± 10%. In many applications the span These can be rounded down to more convenient values of must be adjusted to accommodate individual variations in the 7.5 kΩ and 1.3 MΩ, which will result in an adjustment range sensor as well as the AD693. The span changing resistor should, comfortably greater than ± 200 µA. therefore, include enough adjustment range to handle both the
ADJUSTING INPUT SPAN
sensor uncertainty and the absolute resistance tolerance of P1 Input Span is adjusted by changing the gain of the Signal and P2. Note that the temperature coefficient of the internal Amplifier. This amplifier provides a 0-to-60 mV signal to the resistors is nominally –17 ppm/°C, and that the external V/I section to produce the 4-to-20 mA output span (or a resistors should be comparably stable to insure good tempera- ture performance. REV. A –7–