But there may a situation where we have to deal with external inputs such as analog signals. All most all the input signals from physical sensors are of analog signals. In such cases, we can interface the microcontroller with an external device such as an ADC to convert the analog signal to a digital signal. Because our microcontrollers can only understand 0 and 1. In this article, we look into the details of ADC interfacing with In the present time, there are lots of microcontrollers in the market which has inbuilt ADC with one or more channels, E.
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Resolution: 8 Bits? Single Supply: 5 VDC? Low Power: 15 mW? The converter features a high impedance chopper stabilized comparator, a R voltage divider with analog switch tree and a successive approximation register. The 8-channel multiplexer can directly access any of 8-single-ended analog signals. The device eliminates the need for external zero and full-scale adjustments.
The ADC, ADC 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. Block Diagram Connection Diagrams 1 Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners. Products conform to specifications per the terms of the Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters. The leads should be shorted together or the device placed in conductive foam during storage or handling to prevent electrostatic damage to the MOS gates. DC and AC electrical specifications do not apply when operating the device beyond its specified operating conditions.
LSB 1 Total unadjusted error includes offset, full-scale, linearity, and multiplexer errors. See Figure 5. However, if an all zero code is desired for an analog input other than 0. See Figure LSB 1. The spec allows mV forward bias of either diode. This means that as long as the analog VIN does not exceed the supply voltage by more than mV, the output code will be correct.
The bias current varies directly with clock frequency and has little temperature dependence Figure 8. A particular input channel is selected by using the address decoder. Table 1 shows the input states for the address lines to select any channel. The address is latched into the decoder on the low-to-high transition of the address latch enable signal. Table 1. The converter is designed to give fast, accurate, and repeatable conversions over a wide range of temperatures.
The converter is partitioned into 3 major sections: the R ladder network, the successive approximation register, and the comparator. Monotonicity is particularly important in closed loop feedback control systems.
A non-monotonic relationship can cause oscillations that will be catastrophic for the system. Additionally, the R network does not cause load variations on the reference voltage. The bottom resistor and the top resistor of the ladder network in Figure 3 are not the same value as the remainder of the network. The difference in these resistors causes the output characteristic to be symmetrical with the zero and full-scale points of the transfer curve.
The successive approximation register SAR performs 8 iterations to approximate the input voltage. For any SAR type converter, n-iterations are required for an n-bit converter. Figure 4 shows a typical example of a 3-bit converter. The conversion is begun on the falling edge of the start conversion pulse. A conversion in process will be interrupted by receipt of a new start conversion pulse.
If used in this mode, an external start conversion pulse should be applied after power up. End-of-conversion will go low between 0 and 8 clock pulses after the rising edge of start conversion. It is this section which is responsible for the ultimate accuracy of the entire converter.
It is also the comparator drift which has the greatest influence on the repeatability of the device. A chopper-stabilized comparator provides the most effective method of satisfying all the converter requirements. The chopper-stabilized comparator converts the DC input signal into an AC signal. This signal is then fed through a high gain AC amplifier and has the DC level restored.
This technique limits the drift component of the amplifier since the drift is a DC component which is not passed by the AC amplifier. Resistor Ladder and Switch Tree Figure 4. In ratiometric systems, the physical variable being measured is expressed as a percentage of full-scale which is not necessarily related to an absolute standard.
The voltage input to the ADC is expressed by the equation? The position of the wiper is directly proportional to the output voltage which is a ratio of the full-scale voltage across it. Since the data is represented as a proportion of full-scale, reference requirements are greatly reduced, eliminating a large source of error and cost for many applications. A major advantage of the ADC, ADC is that the input voltage range is equal to the supply range so the transducers can be connected directly across the supply and their outputs connected directly into the multiplexer inputs, Figure Ratiometric transducers such as potentiometers, strain gauges, thermistor bridges, pressure transducers, etc.
This means a system reference must be used which relates the full-scale voltage to the standard volt. The smallest standard step is 1 LSB which is then 20 mV. These voltages are coupled to the comparator via an analog switch tree which is referenced to the supply. The voltages at the top, center and bottom of the ladder must be controlled to maintain proper operation.
The center of the ladder voltage must also be near the center of the supply because the analog switch tree changes from N-channel switches to P-channel switches. These limitations are automatically satisfied in ratiometric systems and can be easily met in ground referenced systems.
Figure 12 shows a ground referenced system with a separate supply and reference. In this system, the supply must be trimmed to match the reference voltage. For instance, if a 5. Figure In Figure 13 a ground referenced system is shown which generates the supply from the reference. The buffer shown can be an op amp of sufficient drive to supply the milliamp of supply current and the desired bus drive, or if a capacitive bus is driven by the outputs a large capacitor will supply the transient supply current as seen in Figure The top and bottom ladder voltages cannot exceed VCC and ground, respectively, but they can be symmetrically less than VCC and greater than ground.
The center of the ladder voltage should always be near the center of the supply. The sensitivity of the converter can be increased, i. In Figure 15, a 2. This system with a 2. The average value of the comparator input current varies directly with clock frequency and with VIN as shown in Figure 8. If no filter capacitors are used at the analog inputs and the signal source impedances are low, the comparator input current should not introduce converter errors, as the transient created by the capacitance discharge will die out before the comparator output is strobed.
If input filter capacitors are desired for noise reduction and signal conditioning they will tend to average out the dynamic comparator input current. It will then take on the characteristics of a DC bias current whose effect can be predicted conventionally. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
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These devices are also designed to operate from common microprocessor control buses, with tri-state output latches driving the data bus. It eliminates the need for external zero and full-scale adjustments. Easy to interface to all microprocessors. It operates on single 5V power supply. Output meet TTL voltage level specifications.
Resolution: 8 Bits? Single Supply: 5 VDC? Low Power: 15 mW? The converter features a high impedance chopper stabilized comparator, a R voltage divider with analog switch tree and a successive approximation register.
8051 – ADC0808 Interfacing
This needs extra hardware and appropriate software, resulting in more complexity and hence the total amount of cost. The circuit of Analogue to Digital converter shown here is configured around ADC, getting rid of the utilization of a microprocessor. It operates on the principle of successive approximation. It features a total of eight analogue input channels, out of which any one can be chosen working with address lines A, B and C.
ADC0808 – Simple Analoque to Digital Converter
ADC Free running Circuit Normally analogue-to-digital converter ADC needs interfacing through a microprocessor to convert analogue data into digital format. This requires hardware and necessary software, resulting in increased complexity and hence the total cost. The circuit of A-to-D converter shown here is configured around ADC , avoiding the use of a microprocessor. It works on the principle of successive approximation.