How to Choose the Best Op Amps

I love precision amplifiers. Most every engineer does. The sweetness of this nearly perfect analog centerpiece component will elicit passion in any good electronics designer.

What wonderful specs do we lust after in our instrumentation or precision amplifier? Offset voltage and bias current are the two big ones in my book. And, darn it, price may be the third most important specification.

We don't care much about slew rate/bandwidth, most of the time, and often low supply current is a second-order need. But, there are many internet of things (IoT) sensor interface applications that demand very low operating and/or standby supply current. Noise is something we do care about, and common mode rejection ratio (CMRR) – sometimes we care a lot about this one. And, common mode range is another thing to keep an eye on. Then there’s power supply rejection ratio (PSRR) and offset voltage drift. That about does it.

Three of the Very Best

The amps we have available are just about miracles. What’s the best offset voltage we can get? The best I’ve seen is 0.1µV typical – and if you want better than that, well, you want an awful lot. And bias current? 1pA is very doable, at room temperature. Quite a few device data sheets don’t spec I_b over temperature – and that’s not good.

Great examples of the very best are three devices from Maxim – the MAX4209, MAX4239, and the MAX9922 (Table 1 provides a comparison of the three op amps):

Table 1: Comparison of three op amps from Maxim.

The MAX4209

The MAX4209 low offset and drift instrumentation amplifier features a very small μMAX package and a spread-spectrum, autozeroing technique that constantly corrects the input offset. The amplifier has rail-to-rail output swing and very high-impedance inputs optimized for small-signal differential voltages (±100mV). It has a gain-bandwidth product of 750kHz and a fixed gain of 100V/V with ±0.03% (typ) accuracy. The device has a reference input to a precision unity-gain buffer for level-shifting the output, allowing for bipolar signals in single supply applications. The MAX4208 version has adjustable gain. They are both specified over the automotive operating temperature range (-40° to 125°C).

Figure 1: The MAX4209 precision instrumentation amplifier is ideal for battery-powered medical equipment, differential voltage amplification, industrial process control, and many other applications.

The MAX4239

The MAX4239 is a low-noise, ultra-high-precision amplifier with 6.5MHz gain-bandwidth and near-zero DC offset and drift using patented autocorrelating zeroing techniques. The decompensated MAX4239 is stable with gains greater than 10 and has a gain-bandwidth product of 6.5 MHz. They feature rail-to-rail outputs and operation from a single 2.7V to 5.5V supply. It requires only 600µA. An active low shutdown mode decreases supply current to 0.1µA. The MAX4238 version is unity-gain stable, with a gain-bandwidth product of 1MHz. They are available in 8-pin narrow SO, 6-pin TDFN, and SOT23 packages.

Figure 2: The MAX4239 ultra-high-precision amplifier is suited for electronic scales, medical instrumentation, thermocouples, and many other applications.

The MAX9922

The MAX9922 high-side current-sense amplifiers feature a +1.9V to +28V input common-mode range that is independent of supply voltage. It is another amp that uses a spread-spectrum autozeroing technique to automatically cancel the input offset voltage. This, in conjunction with the indirect current-feedback technique, achieves less than 10µV (max) offset voltage. The MAX9922 comes in a 10-pin µMAX package and is specified over the -40° to +85°C extended temperature range. The device features a gain accuracy of ±0.06% over temperature. This allows the monitoring of current out of a battery as low as +1.9V, and it enables high-side current sensing at voltages greater than the supply. The MAX9922 has adjustable gain set with two external resistors, while the similar MAX9923 has fixed gains of 25, 50, or 250.

Figure 3: The MAX9922 high-side current-sense amplifier can be used in applications such as handheld Li+ battery current monitoring, notebook/desktop power management, and precision current sources.

All Together

All three of the fine amplifiers above cost around $2 each in 100 quantities. If you have a project that lets you design with one of these great devices, consider yourself lucky – you could be stuck writing software. But, remember, a picoamp is only six thousand electrons/ms. That’s not a lot, so you will have to be really careful with layout and with moisture or other contamination of your circuit.