Figures 9-13 and 9-14 show two typical buffering circuits for the ADCs. Three basic ranges of input signal levels can occur when ADCs are interfaces to the real world. These are as follows: (1) signals that exceed VCC or go below ground; (2) signals with input ranges less than VCC and ground, but are different from the reference range; and (3) signals that have an input range equal to the reference range. Each of these situations requires different buffering. In the last case (in which the signals are equal to the reference), no buffering is usually required, unless the source impedance of the input signal is very high. In this case, a buffer can be added between the multiplexer output and comparator input (Fig. 9-13). An op amp with high input impedance and low output impedance reduces input leakage (when one views the configuration from the multiplexer). If the input signal is within the supply range, but different from the reference range (or when the reference cannot be manipulated to conform to the full input range), the unity-gain buffer of Fig. 9-13 can be replaced with another op amp (as shown in the inset of Fig. 9-13). This type of amplifier provides gain or offset control to produce a full-scale range equal to the reference. When the input range exceeds VCC or goes below ground, the input signals must be level-shifted before the input can go to the multiplexer. There is a limit to such level shifting when the input voltage range is with 5 V, but outside the 0.5-V supply range. In this case, the supply for the entire chip can be shifted to the input range, and the digital-output signals can be level-shifted to the system 5-V supply. A typical example of level-shifting and buffering is the situation in which the bipolar inputs range from -2.5 V to +2.5 V. If the ADCs have the supply and reference provided (as shown in Fig. 9-14), then the ±2.5-V logic outputs can be shifted to 0-V and 5-V SEMICONDUCTOR, APPLICATION NOTE 258, 1994, P. 599.
The ADC0816, ADC0817 data acquisition component is a monolithic CMOS device with an 8-bit analog-to-digital converter, 16-channel multiplexer and microprocessor compatible control logic. The 8-bit A/D converter uses successive approximation as the conversion technique. The converter features a high impedance chopper stabilized comparator, a 256R voltage divider with analog switch tree and a successive approximation register. The 16-channel multiplexer can directly access any one of 16-single-ended analog signals, and provides the logic for additional channel expansion. Signal conditioning of any analog input signal is eased by direct access to the multiplexer output, and to the input of the 8-bit A/D converter.
The device eliminates the need for external zero and full-scale adjustments. Easy interfacing to microprocessors is provided by the latched and decoded multiplexer address inputs and latched TTL TRI-STATE® outputs. The design of the ADC0816, ADC0817 has been optimized by incorporating the most desirable aspects of several A/D conversion techniques. The ADC0816, ADC0817 offers high speed, high accuracy, minimal temperature dependence, excellent long-term accuracy and repeatability, and consumes minimal power. These features make this device ideally suited to applications from process and machine control to consumer and automotive applications. For similar performance in an 8-channel, 28-pin, 8-bit A/D converter, see the ADC0808, ADC0809 data sheet. (See AN-258 for more information.)
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Easy interface to all microprocessorsOperates ratiometrically or with 5 VDC or analog span adjusted voltage reference16-channel multiplexer with latched control logicOutputs meet TTL voltage level specifications0V to 5V analog input voltage range with single 5V supplyNo zero or full-scale adjust requiredStandard hermetic or molded 40-pin DIP packageTemperature range −40˚C to +85˚C or −55˚C to +125˚CLatched TRI-STATE outputDirect access to "comparator in" and "multiplexer out" for signal conditioningADC0816 equivalent to MM74C948ADC0817 equivalent to MM74C948-1
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Supply Voltage (VCC) (Note 3) 6.5V
Voltage at Any Pin −0.3V to (VCC+0.3V)
Except Control Inputs
Voltage at Control Inputs −0.3V to 15V
(START, OE, CLOCK, ALE, EXPANSION CONTROL,
ADD A, ADD B, ADD C, ADD D)
Storage Temperature Range −65˚C to + 150˚C
Package Dissipation at TA = 25˚C 875 mW
Post a Buying Lead
Required Field 
