The AD508 and AD517


ADI entered the super-beta op amp game at an early point, with their own super-beta input part, the AD508, an externally compensated precision device (Reference 15: Modesto Maidique, "A High Precision Super Beta Operational Amplifier," IEEE Journal of Solid State Circuits, Vol. SC-7, December 1972. See also: "AD508: Monolithic Chopperless Op Amp Super β Inputs, <1μV/°C Drift," Analog Dialogue, Vol. 6, No. 1, 1972. (The super-beta input AD508 IC op amp)). Designed by Modesto "Mitch" Maidique who came to ADI from Nova Devices, the AD508 released in 1972, It was an upgrade of his 1971 ADI precision op amp design, the AD504, which was a very high precision op amp in its own right (Reference 16: Modesto Maidique, "Monolithic Operational Amplifier With 1 μV/°C Drift," Analog Dialogue, Vol. 5, No. 3 (The AD504 IC op amp)).

Quite unlike the 108 series of op amps topologically, the AD508 could be said to be an inherently high precision design. It featured the use of thin-film resistors, a super-beta input stage with balanced active loading, and a thermally balanced layout. The design used a two-stage double-integrator topology, with a triply buffered output, for very high load and thermal immunity. The AD508K typically achieved an open-loop gain of ~138dB while driving 2kΩ (much higher than any 108 or 112 topology amplifier), a bias current under 10nA, and a low drift of 0.5μV/°C(max) (Reference 17: Preliminary Data sheet for AD508J, K, L IC Chopperless Low Drift Operational Amplifier, Analog Devices, Inc., June, 1972).

An internally compensated version of the AD508 was introduced in 1978, the AD517 (Reference 18: Doug Grant, "Low Drift Super Beta Op Amp," Analog Dialogue, Vol. 12, No. 1, (The AD517 IC op amp)). This amplifier also used a super-beta input stage, and added the important feature of laser wafer trimming (Reference 19: Richard Wagner, "Laser-Trimming on the Wafer,” Analog Dialogue, Vol. 9, No. 3, (Laser wafer trimming of thin-film resistors for IC offset and gain)). This trimming allowed offset voltage to be held as low as 25μV(max), and drift as low as 0.5μV/°C(max), both for the highest grade, the AD517L.

Much later on, ADI also introduced its own series of internally compensated super-beta op amps, styled along the lines of the OP97 series of devices. These were the AD705 (single), AD706 (dual) and AD704 (quad) series of op amps (References 20-22: “Precision Op Amp," Analog Dialogue, Vol. 24, No. 1 (The AD705 super-beta precision IC op amp); “Two Precision Dual Op Amp Families," Analog Dialogue, Vol. 24, No. 3 (The AD706 and OP297 super-beta precision dual IC op amps); “Quad Op Amp," Analog Dialogue, Vol. 25, No. 1 (The AD704 super-beta precision quad IC op amp)). Designed by Reed Snyder, these op amps were introduced in 1990 and 1991.

Precision Monolithics was purchased by ADI in 1990, and the op amp product lines of the two companies were merged. Today, the product catalog of ADI includes many ADI originated (ADxxx) as well as many original PMI products (OPxxx).

Precision Bipolar IC Op Amps— μA725 to the OP07 families

A second thread of development for precision op amps started at roughly the same time as the LM108 design, in 1969. Working then for Fairchild Semiconductor, George Erdi developed the μA725, the first IC op amp to be designed from the ground up with very high precision in mind.

In a rather complete technical paper on the 725 circuit and precision op amp design in general, Erdi laid down some rules which have become gospel in many terms (Reference 23: George Erdi, "A Low Drift, Low Noise Monolithic Operational Amplifier For Low Level Signal Processing," Fairchild Semiconductor Application Brief APP-136, July 1969 (The uA725 IC op amp)). A simplified schematic of the 725 is shown below in Figure 21.
The μA725 monolithic IC op amp
Figure 21: The μA725 monolithic IC op amp

The 725 is basically a three stage design, consisting of a differential NPN input pair Q1-Q2, followed by a second differential stage, Q7-Q8, and a final single-ended output stage Q22, which is buffered by class AB emitter followers Q21 and Q26. The circuit was externally compensated by a four component RC network at pin 5. The three stages yielded much higher gain than previous two-stage amplifiers, but at the expense of more complex compensation.

Optional trimming of input offset voltage took place at pins 1-8, where an external 100kΩ pot with the wiper to +VS was adjusted for lowest offset. When done in this manner, this also gave lowest drift.
There are some circuit subtleties belied by the schematic's simplicity, but yet important. Q1 and Q2 are actually a quad set (dual pairs), with the paralleled pairs straddling the chip's axis of thermal symmetry. The idea behind this was that thermal changes due to output stage dissipation would be seen as equal thermally induced offsets by the two input stage halves, and thus be rejected. This principle, first established in the 725 design, has since become a basic precision design principle (see Reference 15, again, and within Reference 23, the Fig. 2 chip photograph).

Another key point of 725 performance optimization concerns offset nulling for a condition of zero input offset and lowest drift, described in some detail by Erdi within References 23 and 24(References 24: George Erdi, "Minimizing Offset Voltage Drift With Temperature In Monolithic Operational Amplifiers," Proceedings of NEC, 1969 pp. 121-123 (Method of offset trim for minimum drift in precision DC amplifiers)). The 725 had a typical offset voltage spec of 600μV, and with the offset nulled as recommended, the resulting drift was 0.6μV/°C. The bias current was typically 45nA, and open-loop gain was 132dB.

George Erdi left Fairchild in 1969, to join the newly formed PMI. At PMI, he continued with the 725 precision amplifier concept, designing the SSS725 version (The "SSS" prefix was used on early PMI amplifiers, and stood for Superior Second Source. Another example was the PMI SSS741). This op amp was identical to the original in functionality, but offered improved performance. There was also an OP06 produced at PMI later on. The OP06 was like the 725, but with the addition of differential input protection.

Not too long after the SS725 at PMI came the OP05 op amp, in 1972 (Reference 25: George Erdi, "Instrumentation Operational Amplifier With Low Noise, Drift, Bias Current,” Precision Monolithics APP Note, 1972 (The OP05 IC op amp)). With the new OP05 design George Erdi simplified application of precision op amps considerably, by making it internally compensated, adding input bias current cancellation, and differential overvoltage protection. Topologically, the OP05 can be said to be identical to the 725's three-stage architecture, with these enhancements.

Now, precision op amp users had a simple-to-apply device. But, a major system error was still left to the user to deal with, which was offset voltage. The OP05 used a manual trimming scheme similar to the 725 for offset adjustment, via a 20kΩ pot. The unadjusted maximum offset for the OP05 was 500μV, and drift was 0.6μV/°C after null.

The OP05 was successful in its own right, but the offset voltage issue was still there. About this time, other IC companies were turning to active wafer trim schemes, such as the aforementioned ADI laser wafer trimming scheme (Reference 19, again). The next phase of 725 and OP05 evolution was to address active trimming of op amp offset, so as to deliver higher accuracy in the finished op amp device.

In 1975, Erdi reported on an offset trim technique that used 300mA over-current pulses, to progressively short zener diodes in a string. With the zener string arranged strategically in the input stage load resistances of an op amp, this so-called "zener-zapping" could be used to trim the offset of an op amp on the wafer (Reference 26: George Erdi, "A Precision Trim Technique for Monolithic Analog Circuits," IEEE Journal of Solid-State Circuits, Vol. SC-10, December, 1975 pp. 412-416 (The "zener-zap" offset trim method)). The first op amp to utilize this new trim technique was Erdi's OP07, which was introduced by PMI in 1975 (Reference 27: Donn Soderquist, George Erdi, "The OP-07 Ultra-Low Offset Voltage Op Amp," Precision Monolithics AN-13, December, 1975 (The OP07 IC op amp)).

In the OP07, shown in simplified schematic form in Figure 22, opposite, the (not shown) zener strings are connected in parallel with segmented load resistances R2A and R2B. A simplified schematic of the scheme is shown in Reference 27, Fig. 3, but in essence the series of zener diodes parallel the segmented partial load resistances, the values of which are sized to control progressively larger offsets.

At trim time, a computer measures the actual op amp offset, then selects the appropriate zener to reduce it to the next level, and then zaps that zener with a high pulse of current. This current pulse effectively shorts the zener, and so the section of load resistance in parallel. This process is iterated until the offset cannot be reduced further.

The new OP07 thus created had some impressive offset specifications. It was reported that the entire distribution of parts trimmed had offsets of 150μV or less, and a prime grade, the OP07A was specified at 25μV(max) for offset. Importantly, since this trim method also simultaneously reduced drift as the offset is nulled, the trimmed OP07 amplifiers had drift rates of 0.6μV/°C(max), and typically much less than this.

The zener-zap trim technique was a valuable innovation in its own right, as it could be applied to other devices to reduce errors, and at a low additional cost to the manufacturing process. It is today one of many active trim techniques used with precision op amps (see the more detailed discussions of trimming in Chapter 1).

The OP07 went on to become the "741" of precision op amps, that is the standard device of its precision class. It was (and still is) widely second-sourced, and many spin-off devices followed it in time.
The OP07 monolithic IC op amp
Figure 22: The OP07 monolithic IC op amp

PMI went forward with the OP07 op amp evolution, and introduced the OP77, a higher open-loop gain version of the OP07 in 1988. The best grade OP77A featured a typical gain of ~142dB, an offset of 25μV, and a drift of 0.3μV/°C(max). Later, an additional device was added to the roster, the OP177. This part offered similar performance to the OP77A, as the OP177F, specified over the industrial temperature range.

Prior to the 1990 acquisition of PMI by ADI, the ADI designers turned out some excellent OP07 type amplifiers in their own right. Designed by Moshe Gerstenhaber, the AD707 essentially matched the OP77 and OP177 spec-for-spec, operating over commercial and industrial ranges (Reference 28: “Precision Bipolar Op Amp Has Lowest Offset, Drift,” Analog Dialogue, Vol. 22, No. 1 (The AD707 precision IC op amp)). It was introduced in 1988. The AD708 dual was also offered in 1989, providing basically the performance of two AD707's. Moshe Gerstenhaber also designed the AD708 (Reference 29: Bill Schweber, “Dual Bipolar Op Amp Provides Superior Matching of Specs,” Analog Dialogue, Vol. 23 No. 1 (The AD708 precision IC op amp, dual of the AD707)).

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