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Signal conditioning


Figure 7. Bidirectional current sensing with high common-mode swing. Propagation delay is the key specification for


comparators. Contrary to op amps that get slower when overdriven, comparators will get faster when you overdrive them. Spec tables will sometimes have a propagation delay with a small amount of overdrive, say 5 mV, and a different delay with a larger overdrive of 50 mV or even 100 mV. The number one spec for instrumentation


amplifiers is the common-mode rejection ratio (CMRR). You are trying to extract a very small differential signal riding on top of a large common-mode voltage. Like many specs, this varies with frequency and sometimes a dc CMRR or a CMRR at a very low frequency is listed. A graph of CMRR vs. frequency is usually provided. This would be important, for example, if you were trying to sense current in an H- bridge motor drive as shown in Figure 7. This is probably the most difficult application


for an instrumentation amplifier, because the common-mode voltage goes from near one rail to close to the other, and the current reverses quickly. Gain bandwidth and slew rate are both important.


Programming


Programming in this sense doesn’t mean writing code; it means configuring the part to meet the requirements of your system (although some in- amps do have traditional software programming features with SPI ports and registers). For op amps, we configure the part with


negative feedback. This can be a purely resistive element, but usually a resistor is used with a capacitor in parallel to restrict the bandwidth. This helps the signal-to-noise ratio because noise will be integrated across the entire range, even if we are only using a part of it. You can also use capacitors by themselves and get an integrator or a differentiator. Comparators should always have a bit of


positive feedback to ensure that once the input forces the output to move, the output reinforces the move (see Figure 4 and Figure 5). Some comparators do have internal hysteresis, but you can usually add more if desired. Some comparators with internal hysteresis have a pin to add a resistor to slightly change the amount. It is possible to use an op amp as a comparator,


56


but it is not ideal, and there are several considerations. You must be a good analoger to get away with it in a production environment. Comparators are almost always programmed


with resistors. You can add a high value resistor to give a little bit of positive feedback, and it is also possible to use a capacitor for ac feedback to avoid adding dc hysteresis. Some comparators have built- in hysteresis, but this can be increased, again, by adding a small amount of positive feedback.


Final Considerations


Subtle things happen when trying to use an op amp as a comparator. Quite a few of the low noise bipolar op amps have anti-parallel diodes between the inputs. The input common-mode range for most comparators encompasses 80 per cent of the total range or more. But some low noise, bipolar op amps have one or two diodes in series between the inputs. This is to keep the input stage from Zenering one of the emitter base junctions, which would degrade the noise performance over time. So a 5 V op amp used as a comparator with a


threshold level of 3 V for a power-good indicator in a 3.3 V system would have a problem with 3 V on one input and 0 V on the other, as these diodes limit the maximum differential voltage allowed across the op amp inputs.


summary


For many applications, the choice of op amp will depend on whether you are focused on dc accuracy, ac accuracy, input offset voltage, gain bandwidth, or supply voltage. You have over 700 to choose from. The key parameters for comparators are usually propagation delay and supply voltage. The choice is a little easier, with 122 parts to choose from. The main criterion for instrumentation amplifiers is CMRR as a function of frequency, but near dc, offset voltage, and gain accuracy are also important. Because in-amps are a more specialised part, there are “only” 63 choices. Choosing the right part will result in a


trouble-free, production-worthy design for years to come.


Analog Devices www.analog.com


Access your process dAtA viA Bluetooth


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he PR electronics 4512 Bluetooth communication enabler allows users to enable Bluetooth communication


and data logging on all their PR 4000 and PR 9000 units. Just mount the PR 4512 on the front of the device and install the PPS app on the smart device (iOS and Android). This enables live monitoring of process data, data- and event logging and easy programming of the device. Supporting PR electronics’ existing PR


4000 and PR 9000 devices and all future releases, the 4512 provides the simplest and most cost effective way to enhance functionality of your signal interfaces. Using your smart device, you can now, via


Bluetooth, program and configure your compatible PR devices. You will be able to easily analyse historic and real-time process data on your smart device while on site. With its 100 MB onboard memory, the


4512 can log more than 2.75 million and up to 5.50 million data points (device specific). The 4512 also captures device specific measurement, status or relay errors. Using the built-in real-time clock, it logs the exact time of any error event, incl. relay status. Both logs can be monitored directly in the


PPS app or exported in .csv format. This provides incredible insight into loop performance and optimisation. The 4512 comes with extensive Bluetooth


approvals: USA (FCC), Canada (IC), RED (Radio Equipment directive) for all European countries and Turkey, China (SRRC), South Korea (KC), Australia (RCM), Brazil (ANATEL), Japan (MIC) and Peru (MTC). The device is also approved for mounting


in Zone 2, FM (Div 2), IECEx and FM. www.prelectronics.com


March 2021 Instrumentation Monthly


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