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ADC10062 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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ADC10062 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 14 page Applications Information (Continued) Mode 1 In this mode, the S/H pin controls the start of conversion. S/H is pulled low for a minimum of 250 ns. This causes the comparators in the “coarse” flash converter to become ac- tive. When S/H goes high, the result of the coarse conver- sion is latched and the “fine” conversion begins. After 600 ns (typical), INT goes low, indicating that the conversion results are latched and can be read by pulling RD low. Note that CS must be low to enable S/H or RD. CS is internally “ANDed” with S/H and RD; the input voltage is sampled when CS and S/H are low, and data is read when CS and RD are low. INT is reset high on the rising edge of RD. TABLE 1. Input Multiplexer Programming ADC10064 (a) S 1 S 0 Channel 00 V IN0 01 V IN1 10 V IN2 11 V IN3 ADC10062 (b) S 0 Channel 0V IN0 1V IN1 Mode 2 In Mode 2, also called “RD mode”, the S/H and RD pins are tied together. A conversion is initiated by pulling both pins low. The A/D converter samples the input voltage and causes the coarse comparators to become active. An inter- nal timer then terminates the coarse conversion and begins the fine conversion. 850 ns (typical) after S/H and RD are pull low, INT goes low, indicating that the conversion is completed. Approximately 20 ns later the data appearing on the TRI-STATE output pins will be valid. Note that data will appear on these pins throughout the conversion, but until INT goes low the data at the output pins will be the result of the previous conversion. 2.0 REFERENCE CONSIDERATIONS The ADC10061, ADC10062, and ADC10064 each have two reference inputs. These inputs, V REF+ and VREF−, are fully differential and define the zero to full-scale range of the input signal. The reference inputs can be connected to span the entire supply voltage range (V REF− =0V, VREF+ =VCC) for ratiometric applications, or they can be connected to differ- ent voltages (as long as they are between ground and V CC) when other input spans are required. Reducing the overall V REF span to less than 5V increases the sensitivity of the converter (e.g., if V REF = 2V, then 1 LSB = 1.953 mV). Note, however, that linearity and offset errors become larger when lower reference voltages are used. See the Typical Perfor- mance Curves for more information. For this reason, refer- ence voltages less than 2V are not recommended. In most applications, V REF− will simply be connected to ground, but it is often useful to have an input span that is offset from ground. This situation is easily accommodated by the reference configuration used in the ADC10061, ADC10062, and ADC10064. V REF− can be connected to a voltage other than ground as long as the voltage source connected to this pin is capable of sinking the converter’s reference current (12.5 mA Max @ V REF = 5V). If VREF− is connected to a voltage other than ground, bypass it with multiple capacitors. Since the resistance between the two reference inputs can be as low as 400 Ω, the voltage source driving the reference inputs should have low output impedance. Any noise on either reference input is a potential cause of conversion errors, so each of these pins must be supplied with a clean, low noise voltage source. Each reference pin should be bypassed with a 10 µF tantalum and a 0.1 µF ceramic. 3.0 THE ANALOG INPUT The ADC10061, ADC10062, and ADC10064 sample the analog input voltage once every conversion cycle. When this happens, the input is briefly connected to an impedance approximately equal to 600 Ω in series with 35 pF. Short-duration current spikes can therefore be observed at the analog input during normal operation. These spikes are normal and do not degrade the converter’s performance. Large source impedances can slow the charging of the sampling capacitors and degrade conversion accuracy. Therefore, only signal sources with output impedances less than 500 Ω should be used if rated accuracy is to be achieved at the minimum sample time (250 ns maximum). If the sampling time is increased, the source impedance can be larger. If a signal source has a high output impedance, its output should be buffered with an operational amplifier. The operational amplifier’s output should be well-behaved when driving a switched 35 pF/600 Ω load. Any ringing or voltage shifts at the op amp’s output during the sampling period can result in conversion errors. Correct conversion results will be obtained for input voltages greater than GND − 50 mV and less than V+ + 50 mV. Do not allow the signal source to drive the analog input pin more than 300 mV higher than AV CC and DVCC, or more than 300 mV lower than GND. If an analog input pin is forced beyond these voltages, the current flowing through the pin should be limited to 5 mA or less to avoid permanent dam- age to the IC. The sum of all the overdrive currents into all pins must be less than 20 mA. When the input signal is expected to extend more than 300 mV beyond the power supply limits, some sort of protection scheme should be used. A simple network using diodes and resistors is shown in Figure 4. www.national.com 10 |
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