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ADC0834CCN Datasheet(PDF) 17 Page - National Semiconductor (TI) |
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ADC0834CCN Datasheet(HTML) 17 Page - National Semiconductor (TI) |
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17 / 33 page  Functional Description (Continued) possible because the DI input is only “looked-at” during the MUX addressing interval while the DO line is still in a high impedance state. 3.0 Reference Considerations The voltage applied to the reference input to these convert- ers defines the voltage span of the analog input (the differ- ence between V IN(MAX) and VIN(MIN)) over which the 256 possible output codes apply. The devices can be used in either ratiometric applications or in systems requiring abso- lute accuracy. The reference pin must be connected to a voltage source capable of driving the reference input resis- tance of typically 3.5 k Ω. This pin is the top of a resistor divider string used for the successive approximation conver- sion. In a ratiometric system, the analog input voltage is propor- tional to the voltage used for the A/D reference. This voltage is typically the system power supply, so the V REF pin can be tied to V CC (done internally on the ADC0832). This technique relaxes the stability requirements of the system reference as the analog input and A/D reference move together maintain- ing the same output code for a given input condition. For absolute accuracy, where the analog input varies be- tween very specific voltage limits, the reference pin can be biased with a time and temperature stable voltage source. The LM385 and LM336 reference diodes are good low cur- rent devices to use with these converters. The maximum value of the reference is limited to the V CC supply voltage. The minimum value, however, can be quite small (see Typical Performance Characteristics) to allow direct conversions of transducer outputs providing less than a 5V output span. Particular care must be taken with regard to noise pickup, circuit layout and system error voltage sources when operating with a reduced span due to the increased sensitivity of the converter (1 LSB equals V REF/256). 4.0 The Analog Inputs The most important feature of these converters is that they can be located right at the analog signal source and through just a few wires can communicate with a controlling proces- sor with a highly noise immune serial bit stream. This in itself greatly minimizes circuitry to maintain analog signal accu- racy which otherwise is most susceptible to noise pickup. However, a few words are in order with regard to the analog inputs should the input be noisy to begin with or possibly riding on a large common-mode voltage. The differential input of these converters actually reduces the effects of common-mode input noise, a signal common to both selected “+” and “−” inputs for a conversion (60 Hz is most typical). The time interval between sampling the “+” input and then the “−” input is 1⁄2 of a clock period. The change in the common-mode voltage during this short time interval can cause conversion errors. For a sinusoidal common-mode signal this error is: where f CM is the frequency of the common-mode signal, V PEAK is its peak voltage value and f CLK, is the A/D clock frequency. For a 60 Hz common-mode signal to generate a 1⁄4 LSB error ( ≈5 mV) with the converter running at 250 kHz, its peak value would have to be 6.63V which would be larger than allowed as it exceeds the maximum analog input limits. Due to the sampling nature of the analog inputs short spikes of current enter the “+” input and exit the “−” input at the clock edges during the actual conversion. These currents decay rapidly and do not cause errors as the internal com- parator is strobed at the end of a clock period. Bypass capacitors at the inputs will average these currents and cause an effective DC current to flow through the output 00558357 a) Ratiometric 00558358 b) Absolute with a reduced Span FIGURE 2. Reference Examples www.national.com 17 |
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