Sample and Hold Circuit and Multiplexer

Sample and Hold Circuit and Multiplexer

AU : May-04, 05, Dec.-05

A sample and hold circuit is used to interface real-world signals. The purpose of this circuit is to hold the analog value steady for a short time while the converter or other following system performs some operation that takes a little time.

For accurate analog to digital conversion the analog input voltage should be held constant during the conversion cycle. If the analog input voltage changes by more than ± 1/2 LSB an error can occur in the digital output code. To illustrate the effect of a changing analog input voltage on the conversion processor, let us consider a situation of a successive approximation ADC with an analog input voltage that is initially zero, but there happen to be a large change in voltage amplitude occurring during the conversion process. Fig. 13.9.1 shows the changing input voltage and its effect on the successive approximation conversion process.

As shown in Fig. 13.9.1 analog input voltage at start of conversion process is zero volts and at the end of conversion process it is near to 1.5 volts, and the conversion process result is 010₂, i.e. 2.5 V. This result does not corresponds to the analog voltage at the start of conversion or at the end of conversion. To minimise the occurrence of these errors it is necessary to hold the value of the analog input voltage constant during the conversion process. The sample and hold circuit does this task.

As its name implies, the sample and hold (S/H) circuit samples the value of the input signal in response to a sampling command and hold it at the output until arrival of the next command.

It samples an analog input voltage in a very short period, generally in the range of 1 to 10 µs, and holds the sampled voltage level for an extended period, which can range from a few millisecond to several seconds. Fig. 13.9.2(b) shows input and output response of the sample and hold circuit.

The sample and hold circuit uses to basic components analog switch and capacitor. The Fig. 13.9.2(a) shows the basic sample and hold circuit. The circuit tracks the analog signal until the sample command causes the digital switch to isolate the capacitor from the signal, and the capacitor holds this analog voltage during A/D conversion.

The Fig. 13.9.3(a) illustrates a practical data acquisition system using an analog multiplexer, a sample and hold circuit and an A/D converter. In such system, microprocessor selects one of the input channels through multiplexer address lines. When the  / hold signal is low, the sample and hold circuit tracks the analog input; otherwise the sample and hold circuit holds the analog input. The stable analog signal is then converted into its digital equivalent using A/D converter. Microprocessor reads this output using input port.

1. Data Acquisition System using 8085

The Fig. 13.9.4 shows the data acquisition system using 8085 and 8255. Here 8255 is used to generate required control signals and accept data from A/D converter.

According to ports used we require to configure 8255 as follows :

• Port A : input (mode 0)

• Port B : input (mode 0)

• Port C : output

Control Word

Program : To take from analog channel 2.

MVI A, 92H ; Configure 8255 by

OUT 03H ; Sending control word to control register

MVI A, 22H ; Send MUX address, make  /hold

OUT 02H ; line HIGH and make SOC low

MVI A, 0AH ; Make   /hold Low using BSR mode

OUT 03H  

MVI A, 0BH ; Make   /hold High using BSR mode

OUT 03H  

MVI A, 09H ; Make SOC High using BSR mode

OUT 03H  

MVI A, 08H ; Make SOC low using BSR mode

OUT 03H  

BACK : IN 01H ; Read port B

ANI 01H ; Check PB₀

JZ Back ; if zero wait for Data Ready

IN 00H ; Read data through port A

HLT ; Stop

2. Another Way of Connecting MUX and SH Circuits

In the previous examples we have seen that sample and hold circuit is connected after the multiplexer. Here, we have to select analog signal turn by turn and then sample it. Thus, we can not sample all the channels at the same time. If it is required to read values from all the channels at the same time then we need separate sample and hold circuit for each channel. This is illustrated in Fig. 13.9.5. With this configuration we can sample analog signals from all channels at a time. The microprocessor then selects channel one by one and read the respective values one by one with the help of multiplexer and A/D converter.

Review Questions

1. What is sample and hold circuit ? Explain the purpose of it.

2. Explain the use of sample and hold circuit with multiplexer.

3. Draw and explain the data acquisition system.

4. Draw and explain the operation of a sample and hold circuit. AU : May-04,05, Marks 4

5. Explain the sample and hold IC.  

AU : Dec.-05, Marks 12