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Abstract: This article discusses application circuits for Maxim force/sense digital-to-analog converters (DACs). Applications include: selectable fixed-gain DAC, programmable gain DAC, photodiode bias control, amperometric sensor control, digitally programmable current source, Kelvin load sensing, temperature sensing, and high current DAC output. A brief description of the various DAC output configurations is also given.
Force/Sense DACs are unique because they provide user access to the inverting node of the output buffer amp in addition to the conventional output. These DACs are interesting because they provide flexibility to create custom DAC gains, or other useful circuits by simply adding a few simple components.
Types of Buffered, Voltage DAC OutputsFigure 1 shows three common types of buffered, voltage output DACs. The first has a fixed-gain defined by an internal resistor ratio (typically +1.0, +1.638, or +2.0V/V), and no provision for adjusting the offset. The second type also uses internal resistors to set a fixed gain, but the normally grounded resistor tap in the non-inverting gain op amp topology is brought to an external pin allowing offset adjustment. The final output type is force/sense, which provides a pin connecting directly to the inverting terminal of the output op amp, yielding the most flexibility.
Figure 1. DAC output types: (a) fixed-gain without offset adjustment, (b) fixed-gain with offset adjustment, (c) force/sense.
The main advantage of the first two DAC types (Figure 1a and 1b) is the internal resistors are trimmed to provide typical gain error lower than ±1%, and they track each other closely over temperature to provide a typical gain tempco below 10ppm. The drawback is a single, fixed-gain that can src="/data/attachment/portal/201007/ET12534201007191111332.gif">
Figure 2. DAC with selectable fixed-gain of +2.20V/V.
Digitally programmable DAC gains are also possible by combining a force/sense DAC and digital potentiometer(s). Two examples of this using the MAX5175/MAX5177 DACs and MAX5400/MAX5415 digital potentiometers are shown in Figure 3. Both parts share the same SPI interface allowing write-only functionality to be implemented with four digital interface signals (clock, data input, and two chip selects).
Figure 3. DAC with programmable gains using a digital potentiometer: (a) larger gain range, lower gain setting resolution, (b) smaller gain range, higher gain setting resolution.
In the first circuit (Figure 3a), the gain of the MAX5175 is set by a single MAX5400 digital potentiometer, defaulting to a gain of +1.992V/V at power-up. The gain tuning resolution is ~±0.8% around this default value, indicating a gain of +2.00V/V can be set within ~±0.4%. The non-linear gain range spans approximately +1V/V to +255V/V, although the upper usable gain will be limited by the reference and supply voltages. The gain setting resolution also worsens at higher gains. src="/data/attachment/portal/201007/ET12534201007191111334.gif">
Figure 4. Bias voltage control of a photodiode and transimpedance amplifier: (a) grounded reverse bias with single DAC, (b) level shifted zero or reverse bias with dual DACs.
Another transimpedance application is shown in Figure 5. In this example, the DAC provides a DC voltage bias for an amperometric sensor, and the sensor's output current is converted to a voltage by the DAC's transimpedance amplifier. Amperometric (or more generally voltammetric) sensors are commonly used in medical applications, and the force/sense DAC is a natural fit.
Figure 5. Voltage bias control for an amperometric sensor.
A force/sense DAC can also be configured as a digitally programmable current source (actually a sink) using the topology shown in Figure 6. Assuming the DAC output has enough headroom to drive the ~0.7V base-emitter voltage of the NPN BJT, feedback will hold the voltage across the resistor (and hence its current) constant at the unbuffered R-2R ladder output voltage(VR2R). The output current at the BJT's collector will be slightly lower than the programmed resistor current due to the BJT's finite beta. This may require calibration of the output current in some applications. src="/data/attachment/portal/201007/ET12534201007191111336.gif">
Figure 6. Digitally programmable current source.
By using Kelvin sensing, force/sense DACs are able to deliver a desired voltage at the load, even if the series impedance between the DAC output and load is relatively high. src="/data/attachment/portal/201007/ET12534201007191111337.gif">
Figure 7. Driven signal with kelvin sensing at load.
Several products are now available that use discrete diode or transistor P-N junctions for remote temperature sensing. These devices use ratios of 2 (or more) currents along with the diode equation to determine the temperature in Kelvin.
Figure 8 shows a simple circuit topology where a MAX5302 DAC is used to drive a diode-connected transistor. Currents (1023 possible values) are set by the DAC output voltage across the grounded resistor (VR/R), and the resulting forward-biased, P-N junction voltage (VD) is measured differentially using a MAX1408 ADC. This topology is convenient because the same resistor is used to set both currents, and the current ratios are approximately the ratios of DAC code words. For example, DAC codes of 1000 (decimal) and 500 (decimal) produce a current ratio of 2:1 that can be used directly in the diode equation calculations. There will be a few DAC error terms such as offset, INL, and gain affecting the ratio, but these will be relatively minor. If more accuracy is needed, the precise current ratio can be determined by using the ADC to measure the forced resistor voltage (VR) at each DAC code. Care should be taken with this circuit to ensure op amp stability, especially for distant P-N junctions with long leads and large parasitics.
Figure 8. Diode current drive for temperature sensing.
The final application circuit shown in Figure 9 is a MAX5352/53 force/sense DAC with an emitter-follower, BJT stage to increase output current drive. This topology is similar to the current source shown in Figure 6, except the collector is tied to VCC (common-collector) and the output is a voltage at the emitter rather than current at the collector. As with many previous circuits, a key requirement is that the DAC output have enough headroom to drive the BJT's base-emitter junction ~0.7V above the maximum output voltage.
Figure 9. Increased DAC output current drive with an emitter-follower.
Maxim Force/Sense DACsTable 1 lists all of the force/sense DACs currently offered by Maxim. A few key specifications are also shown.
Table 1. Maxim Force/Sense DACs
Maxim DACConfigurationResolutionSupplyReferenceMAX5304Single10 Bits |
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