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Sensor, signal conditioning define the real performance envelope

I'm a little envious: I see lots of attention given to processors, system architectures, IoT wireless connectivity, development software and tools, GUIs, embedded processors, ultra-low power anything and everything – you catch my drift. Yet for many of their intended applications, success starts with a sensor and its signal conditioning front end (often called the AFE—analog front end). In some cases, such as a basic temperature measurement to ±0.5⁰C, you can get away cheap and easy by using a diode or thermistor and a slow but high-resolution A/D converter which is in the microcontroller itself.

But that’s just for some cases. Today's higher-end instrumentation needs a good sensor, and equally important, a correspondingly good AFE. You can dress up the GUI with as many bells and whistles as you want, but if the sensor is inadequate or the AFE doesn’t do it justice, you've got a pretty picture and not much else. Even more numeric processing won't help because, in most cases, the algorithms can’t improve the basic accuracy and precision of the transducer and its AFE (although they may be able to do something about noise by averaging or curve fitting).

The AFE problem has become more challenging in recent years, for several reasons. First, users are demanding increased accuracy in measurements, with parts per billion (ppb) resolution replacing parts per million (ppm). Second, many of these instruments are in unattended, remote, or outdoor locations, so temperature drift of the electronics is a major factor. Third, some of the most exciting and innovative sensing design involves subtle optical phenomena and transducers such as for Raman spectroscopy, and the AFE must be matched to the unique and often complex characteristics of these devices.

Ironically, cost is not the major factor in many of these designs (how often do you hear that?) despite the standard disclaimer “we're in a very cost-competitive market.” For many of these instruments, the total BOM value is quite high, and another few dollars on the AFE to ensure leading-edge performance is a good use of the budget. Further, many of them have just one or two sensor channels, so the actual impact a higher-price AFE is moderated (yes, there are exceptions to that statement, such as esoteric physics research at CERN's Large Hadron Collider or the IceCube Telescope at the South Pole).

All these factors came together when I saw a new electrometer op amp from Analog Devices, the ADA4530-1. To quote the press release (and you are warned: this is mouthful) the device is “ideal for interfacing to sensors that are sensitive to output loading such as photo diodes and other high output impedance sensors often used in precision monitoring/analysis equipment such as spectrophotometers, chromatographs and mass spectrometers, as well as potentiostatic and amperostatic coulometry measurement devices. The new amp also can be used as a front-end amplifier for picoammeter and coulombmeter instrumentation systems, as a transimpedance amplifier for photodiodes, ion chambers, and working electrode measurements, or as a high-impedance buffer for chemical and capacitive sensors.”

Whew, that's serious instrumentation, absolutely. Two primary attributes make this component especially attractive: it has very low bias current of 20 fA (maximum) at 25°C, and that current rises to only 250 fA (maximum) at 125o C – and those specifications are 100% tested, not just promised, Figure 1 . (If you don’t know why low bias current is critical for these applications, it's worth your time to investigate the world of scientific and advanced electrometer sensors, extremely high-impedance sources which deliver minute amounts of current, not voltage.)

Figure 1

The bias current of the ADA4530-1 is a low 20 fA at 25°C, and increases by a little over 10× at 125°C, which makes it a suitable for instrumentation in harsh temperature environments that also must maintain precision performance.

The bias current of the ADA4530-1 is a low 20 fA at 25°C, and increases by a little over 10× at 125°C, which makes it a suitable for instrumentation in harsh temperature environments that also must maintain precision performance.

Even using a component such as this is also a challenge: you may need to go to Teflon standoffs and a super-clean PC board, as standard PC board material (FR-4) may have too much leakage current across its surface. The ADA4530-1 also provides an output for driving PCB “guard rings” or the guard shield of interconnecting shielding, which is used to reduce the effects of stray capacitance on the sensitive connections between the sensor and the amplifier. Again, this is very serious sensor-related circuit design, where small deviations from special practice can add up to large errors in the signal, often seen as noise, inconsistency, and other headaches. Given what an electrometer op amp has to do to ensure precision performance, the $11 price is actually very reasonable – even though it is a long away from those “commodity” parts which go for well under a dollar.

Have you ever had to spend substantially more for a top-performance part, in order to get the specifications you need?

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