Precision op-amp bargains

Much in demand these days is the precision or instrumentation amplifier. Used for grabbing small signals from a sensors and detectors, they have become pretty cheap and very high performance — a nice combination.

OK, so I'm going to make the rule that to be in the “precision” category you gotta have an input offset voltage of less than 200 µV and input bias current less than 200 pA. The amplifier must also have offset drift over temperature of less than 1 µv/°C and a CMRR of 95 dB or better. I decided to look for singles only — no duals or quads — just to narrow things down. We do not care about gain/bandwidth product or slew rate for this application. I found five choice devices we can review.

First we have the LMP2231 from Texas instruments, which has an offset of less than 150 µV at room temperature, a drift of 0.3 µV/°, a CMRR of 97 dB. Bias current is a skimpy 50 pA over temperature; 0.02 pA typ. Not bad, not bad at all — especially for only 73 cents (at 1k). And it operates over -40° to 125°C and comes in SOT-23 or SOIC packages. The bandwidth for this device is 130 kHz and supply current is only 10 µA, with a supply voltage range of 1.6 to 5.5 V.

TI's TLC2652A  advanced LinCMOS chopper-stabilized amp features an offset of just 1 µV with drift of just 0.003 μV/°C typ along with a low-input-bias current of 4 pA typ and 100 pA max over temperature. The chip's CMRR is 120 dB minimum. The part's common-mode input voltage range includes the negative rail. Supply current is 1.5 mA typ and 2.5 mA max. Now, this one is a bit expensive, costing $2.30 ea/1,000. It come in small-outline, ceramic DIP, and plastic DIP packages.

The MAX9617 from Maxim Integrated is available in a space-saving 2 x 2-mm, 6-pin SC70 package and features 10 μV max input offset voltage, 120 nV/°C max offset drift, and input bias current of 80 pA max at 25°C and 95 pA up to 85°C. CMRR is 116 dB min and 135 dB typ, and the gain bandwidth product is a speedy 1.5 MHz. It will cost you $1.10 ea/2,500.

Linear Technology offers the fine LTC2054, which has guaranteed specifications from -55° to 150°C. Input offset voltage is just 3 μV max at -40° to 85°C, and offset voltage drift is 30 nV/°C max.The amp has a CMRR of 115 dB minimum, input bias current of 1 pA (typ, 25°C) and 150 pA max over temperature. The chip starts at  1.17 ea/500 and it is available in a a number of packages, including a 3 × 3 × 0.8-mm dual fine-pitch leadless package (DFN). Supply current is just 150 μA max, and gain bandwidth is 500 kHz.

Last is the excellent ADA4051-1 from Analog Devices, which has an input offset of 2 µV typ and 17 µV max and a offset drift of 0.02 µV/°C typ. CMMR is 125 dB typ and input bias current is 5 pA typ and 50 pA max, with 200 pA max over −40° to 125°C. It also has single-supply operation from 1.8 to 5.5 V with just 13-μA typ supply current. Gain bandwidth is a leisurely 115 kHz. Price is from $1.12 ea/10,000.

There are a great many “precision” or “instrumentation” amplifiers available, but the ones above will certainly give you a good starting point when searching for that optimum part.