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PCM78P Fiches technique(PDF) 14 Page - Burr-Brown (TI) |
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PCM78P Fiches technique(HTML) 14 Page - Burr-Brown (TI) |
14 / 16 page ® PCM78 14 decoupling capacitor should range from 0.1 µF to 4.7µF; larger values can cause reference settling problems which may manifest themselves as missing codes. This capacitor should be as close to the PCM78 as possible, to minimize the potential for coupling noise into the device; with a good board layout it may be best to leave this capacitor out of the circuit altogether, as the extra lead length may only cause more noise in the reference. Pin 27 is a decoupling point to ground, as well as the output of the 2V reference. This point should not be used to supply reference voltage to external circuitry unless it is buffered. A 2.2 µF capacitor is recommended, and the capacitor used here should not exceed 4.7 µF. Pin 28, the Speed Up pin, allows a capacitor to be connected to ground to facilitate reference settling. This does not speed up the conversion time, but it does reduce odd order harmonic distortion. As with the decoupling capacitor on pin 25, this may also contribute to noise; if harmonic content is most important in an application, this capacitor (0.1 µF - 10 µF) should be connected. In all other cases, it is best to leave the capacitor out of the circuit. Input Scaling The analog input should be scaled as close to the maximum input signal range as possible in order to utilize the maximum signal resolution of the A/ D converter. The DAC inside the PCM78 has a ±2mA range, and the nominal ±3V input is scaled by a 1.5k Ω resistor. In order to scale to other ranges, see Table I for recommended scaling resistor values, connected as shown in Figure 14. INPUT RANGE R ±10V 8.2k Ω ±5V 3.3k Ω NOTE: R values shown assume use of 1k trim pot to adjust for scale accuracy. TABLE I. PCM78 Input Scaling Resistor Values. If Short Cycle is not held low until the next convert command is issued, the Status line will go high in synchronization with Short Cycle. This is because the operation of the Status line becomes invalid after Short Cycle is asserted. An example of the Short Cycle operation is shown in Figure 12. In those systems where a user may not be using a continuous external clock, it is necessary to assure that a falling edge of external clock occurs after short cycle goes low. This is because conversion actually stops on the first falling edge of external clock after Short Cycle goes low. ANALOG CIRCUIT CONSIDERATIONS Layout Precautions Analog and Digital Common are connected internally in the PCM78, and should be connected together as close to the unit as possible, preferably to a large ground plane under the ADC. Low impedance analog and digital common returns are essential for low noise performance. Coupling between analog inputs and digital lines should be minimized by careful layout. The input pin (pin 1) and the MSB adjust pin (pin 3) are both extremely sensitive to noise; digital lines should be kept away from these pins to avoid coupling digital noise into the sensitive analog circuitry. Contact factory for a recommended PCB layout for the PCM78. Power Supply Decoupling The power supplies should be bypassed with tantalum or electrolytic capacitors as shown in Figure 13 to obtain noise free operation. These capacitors should be located as close to the ADC as possible. Bypass the 1 µF electrolytic capacitors with 0.01 µF ceramic or polystyrene capacitors for improved high frequency performance. +V .01µF + + 1µF CC –V 1µF + + .01µF CC + .01µF 1µF +V DD PCM78 2 4 23 FIGURE 13. Recommended Power Supply Decoupling. Reference Decoupling and Speed Up In order to assure the lowest noise operation of the PCM78, the reference may be bypassed by three different capacitors. Pin 25 is a decoupling point for the reference to –VCC. The *1k Ω 1 PCM78 R *Use to trim for exact scaling. Use trim pot with temperature coefficient of 100ppm/°C or better. FIGURE 14. PCM78 Input Scaling Circuit. INPUT IMPEDANCE The input signal to the PCM78 should come from a low impedance source, such as the output of an op amp, to avoid any errors due to the dynamic input impedance that a successive-approximation converter presents to the outside world because of the changing currents in this circuit during conversion as the converter steps through its approxima- tions. If the driving circuit output impedance is not low, a buffer amplifier should be added between the input signal and the direct input to the PCM78 as shown in Figure 15. |
Numéro de pièce similaire - PCM78P |
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Description similaire - PCM78P |
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