EKT 314/4 WEEK 10 : CHAPTER 4 DATA ACQUISITION AND CONVERSION ELECTRONIC INSTRUMENTATION

Chapter 4 Content Conditioning Single Channel Data Acquisition Systems Multi Channel Data Acquisition Systems Data Conversion Multiplexers Basic Concept of Multiplexer Mux-Demux Digital Multiplexer Sample and Hold

Basic Concept of Multiplexer In electronics, a multiplexer or mux is a device that performs multiplexing It selects one of many analog or digital input signals and forwards the selected input into a single line. A multiplexer of 2 n inputs has n select lines, which are used to select which input line to send to the output. Demultiplexer works in the opposite way

Basic Concept of Multiplexer Multiple Input Pin Single Ouput Pin Input Selector Pin 1 pin = max 2 input 2 pin = max 4 input 3 pin = max 8 input N pin = max 2 n input Output Enable Pin sometimes available.

Mux-Demux One use for multiplexers is cost savings by connecting a multiplexer and a demultiplexer (or demux) together over a single channel (by connecting the multiplexer's single output to the demultiplexer's single input). Often, a multiplexer and demultiplexer are combined together into a single piece of instrument, which is usually referred to simply as a "multiplexer". Both pieces of equipment are needed at both ends of a transmission link because most communications systems transmit in both directions. In analog circuit design, a multiplexer is a special type of analog switch that connects one signal selected from several inputs to a single output.

Digital Multiplexer In digital circuit design, the selector wires are of digital value. A 4-to-1 multiplexer has a boolean equation where and are the four inputs, I 0 to I 3, S 0 and S 1 is the selector input, and F is the output.

Chapter 4 Content Conditioning Single Channel Data Acquisition Systems Multi Channel Data Acquisition Systems Data Conversion Multiplexers Sample and Hold Sample and Hold Amplifier (SHA) SHA Component Basic SHA Operation

Sample and Hold Amplifier (SHA) A critical part of many data acquisition systems It captures an analogue signal and holds it during some operation (most commonly during analogue-to-digital conversion). The circuitry involved very demanding. Unexpected properties of commonplace components (cap, pcb) may degrade SHA performance. Integral SHA introduction in ADC kill this concern.

SHA Component Input Amplifer Energy storage device Output buffer Switching circuit

SHA Component Input Amplifer Buffers the input Present high mpedance to signal source Provide current gain to charge hold capacitor Energy storage device Output buffer Switching circuit

SHA Component Input Amplifer Energy storage device Heart of SHA Almost always a capacitor In track mode, the voltage in the hold cap follows the input signal In hold mode, the switch is opened, and the capacitor retains the voltage present before it disconnected from input buffer. Output buffer Switching circuit

SHA Component Input Amplifer Energy storage device Output buffer Switching circuit

SHA Component Input Amplifer Energy storage device Output buffer Offers a high impedance to the hold cap to keep the held voltage from discharching prematurely Switching circuit

SHA Component Input Amplifer Energy storage device Output buffer Switching circuit Form the mechanism by which the SHA is alternately switched between track and hold.

Basic SHA Operation Track mode Function simply same as amplifier Principle spec: offset, gain, non-linearity, bandwidth, slew rate, settling time, distortion, and noise. Track-to-hold transition Hold mode Hold-to-track transition.

Basic SHA Operation Track mode Track-to-hold transition Generally a small amount of charge is dumped on the hold cap because of non-ideal switch. Result in hold mode offset voltage (called pedestal error) Pedestal error may be reduced by increasing the value of the hold cap Switching from track to hold produce transient – hold mode settling time. Hold mode Hold-to-track transition.

Basic SHA Operation Track mode Track-to-hold transition Hold mode During this mode there are error due to imperfection of hold cap, switch and output amplifier. If a leakage current flows in or out of the hold cap, it will change the voltage – effect known as droop. Leakage current in PCB may be minimized by using guard rings. Hold-to-track transition.

Basic SHA Operation Track mode Track-to-hold transition Hold mode Hold-to-track transition. Must reacquire input signal Acquisition time involved Output buffer settling time must be significantly smaller than the hold time.

EKT 314/4 WEEK 10 : CHAPTER 4 END ELECTRONIC INSTRUMENTATION