Control System Basics – Isolation

In the final installment of this five-part series, Jon Titus explores the basic elements of a control system. In previous installments, Jon has covered relays, sink vs. source control, PNP vs. NPN logic and open collectors. Q: I don’t like the idea of control signals going from a field device into my rack of expensive […]

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Analog Resolution vs. Accuracy

  Q: I need an analog-to-digital converter, probably one with 16 bits. The more bits, the better the resolution, right? A: Not so fast. To start, understand the difference between resolution and accuracy. I have an inexpensive 6-inch plastic ruler and a steel machinist’s ruler. Both have a resolution of sixteenths of an inch, but […]

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Analog Signal Conditioning for Accurate Measurements

  Q: Should I put some sort of circuit between my sensor and an analog-to-digital converter? A: Yes. You probably need some signal conditioning. The explanation below goes on for a bit, but stay with it and you’ll understand what you need and why you need it. Before you make any connections, get the electrical […]

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Basics of RS-422 and RS-485 Communications

  Q: I must create an instrument network but need some guidance on RS-422 and RS-485 communications. What’s the difference? A: First, carefully describe what you want the network to achieve and then examine the physical-layer characteristics of each standard. Here “physical layer” refers to the signals on the bus or buses, signal timing, and connections […]

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Control System Basics – Open Collectors

In the fourth installment of this five-part series, Jon Titus explores the basic elements of a control system. In previous installments, Jon has covered relays, sink vs. source control and PNP vs. NPN logic. Check back next month for the final installment. Q: It looks like NPN and PNP sensors each provide an “open” collector […]

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Control System Basics – PNP vs. NPN Logic

In the third installment of this five-part series, Jon Titus explores the basic elements of a control system. In the first two installments, Jon covered relays and sink vs. source control. Check back next month for the 4th installment. Q: Now I understand relays and current sinks and sources. Some of the sensors I plan […]

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Control System Basics – Relays Explained

In the first installment of this five-part series, Jon Titus explores the basic elements of a control system. Check back next month for the 2nd installment. Q: I just started to design a control system and plan to use relays to turn devices on or off. Relay-module specifications note “C relays” and NO and NC […]

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Control System Basics – Sink vs. Source Control

In the second installment of this five-part series, Jon Titus explores the basic elements of a control system. Click the following link if you missed the first installment: Relays Explained. Check back next month for the 3rd installment. Q: OK, the relays and their operations seem clear. In a few cases I’ll use an off-the-shelf […]

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Determining an Effective Analog Sampling Rate

  Q: How often should my equipment make measurements? A: This question often arises when people draft plans to automatically measure a physical quantity such as temperature, pressure, acidity, liquid level, and so on. You can approach this problem in several ways, from an educated guess to a mathematical analysis of your system. The examples that follow […]

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Differential Nonlinearity in Analog Measurements

  Q: In a previous answer you noted, “… most applications require linearity but not absolute precision…” What does that mean? A: Typically it means differential nonlinearity, or DNL. The DNL information indicates the difference between the measured difference between a perfect change of one LSB voltage weight between two adjacent codes in the ADC. […]

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Getting the Max Resolution from Your Analog Measurements

  Q: How do resolution and accuracy relate to an ADC? Do they mean the same thing? A: No, each term refers to something different. Say I have an inexpensive plastic ruler with a centimeter scale marked at 1-millimeter increments. That scale gives me a 1-mm resolution. So I can measure, or resolve, a length […]

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