The AD711/712/713 and OP249 IC Op Amps

In 1986 the AD711/712 and AD548/648 FET op amp families were introduced by ADI (Reference 46: "Highest-Performing Low-Cost BiFET Op Amps," Analog Dialogue, Vol. 20, No. 2, 1986, pp.22.). The AD711/712 were, respectively, single and dual parts with finely tuned specifications, designed to meet both general purpose as well as intermediate precision uses, but at a moderate cost. The AD712KN sold for $1.90 in quantities of 100, while the AD648KN sold in similar lots for $2.60.

The series featured offset voltages of 500μV(max), a drift of 10μV/°C(max) for the AD711K, at a quiescent current of 3mA. The AD548K had similar offset voltage specifications, and half the drift, at a supply current of 200μA. JoAnn Close designed the AD548/648 series of amplifiers, with inputs from Scott Wurcer and Lew Counts. Scott Wurcer designed the AD711/712 series. The AD711 and AD712 were ultimately to be joined by a quad version, the AD713. This family of JFET IC op amps have been very popular since their introduction, and are still available.

Prior to the 1990 acquisition by ADI, PMI introduced their own dual JFET input IC op amp, the OP249. Designed by Jim Butler, this similarly specified dual op amp competed directly against the AD712.

Electrometer IC Op Amps

amplifier, where input currents are required to be less than 1pA. In the days of the modular op amp, such ultra low current devices as the model 310 and 311 varactor bridge amplifiers had addressed this role. See the previous section of this chapter for a basic discussion on these amplifiers. It should be understood that the term electrometer amplifier is here meant to imply any amplifier with ultra low bias currents. It might be a varactor bridge based design, or it might be some other type of front end allowing ultra low bias currents, for example several semiconductor types— MOSFETs, JFETs, etc.

The AD515 and AD545 Hybrid IC Electrometer Amplifiers

In hybrid IC form, there were a couple of early electrometer op amps from ADI. The first of these was the AD515, a two-chip hybrid similar in general architecture to the AD503 (discussed in conjunction with Fig. 25). The AD515 operated at a low power, with a quiescent current of 1.5mA (Reference 47: Dave Kress, "FET-Input Electrometers," Analog Dialogue, Vol. 10, No. 1, 1976, pp.11). It achieved some impressively low input currents; 75fA for the best grade AD515L, while maintaining a low offset of 1mV(max). The AD515 was a successful product, with specifications that were not soon to be eclipsed.

Another early two-chip hybrid IC electrometer op amp was the AD545, introduced in 1978 (Reference 48: "FET-Input AD545," Analog Dialogue, Vol. 12, No. 3, 1978, pp.18). This design also operated at low power like the AD515, but with a higher maximum input bias current, 1pA for the AD545L.

Monolithic IC Electrometer Amplifiers

One of the early monolithic IC electrometer op amps, was the OPA111. Burr-Brown introduced this device in 1984 (Reference 49: Steve Millaway, "Monolithic Op Amp Hits Trio of Lows," Electronic Design, February 9, 1984 (The OPA111 low bias current IC op amp)). Designed by Steve Millaway, the OPA111 used a dielectrically-isolated process for fabrication.

The OPA111 circuit employed P-channel JFETs in the input and second stages, and a first stage cascode design for low bias current variation with input CM changes. The design addressed some of the weak points of the previous LF155/156/157 series (Reference 39, again). Reference 49 cited several LF15x circuit weaknesses; one was the use of current source loading for the input JFET pair, another was the means of offset trimming, and another was potential susceptibility to popcorn noise, due to the noise currents of the second stage bipolar differential pair. These points were addressed by the OPA111 design.

The OPA111 name was said to have been based on the combination of three key specs; 1mV(max) offset, a drift of 1μV/°C(max), and an input voltage noise of 1μVrms in a 10Hz-10kHz bandwidth. This particular combination of specifications was tough to beat, and the OPA111 became a successful IC op amp.

Released in 1987, the first completely monolithic IC electrometer op amp from ADI was the AD549, designed by JoAnn Close and Lew Counts (Reference 50: JoAnn Close, Lew Counts, “Junction-Isolation Process Yields 50-fA Op Amp," Electronic Design, July 10, 1986, pp.99-102, 104. See also: JoAnn Close, "Monolithic Electrometer Has 60fA Max Bias Current," Analog Dialogue, Vol. 21, No. 2, 1987, pp. 22 (The AD549 electrometer IC op amp)). This op amp achieved its low bias current by virtue of the use of a new "topgate" FET, as designed by Jody Lapham and Paul Brokaw (Reference 51: Jerome F. Lapham, Adrian P. Brokaw, "Low-Leakage JFET," US Patent 4,985,739, filed April 27, 1987, issued January 15, 1991. (Design of the ‘top-gate’ low-leakage JFET IC process)), plus a sophisticated scheme of bootstrapping around the critical input P-channel JFET pair.

A schematic as adapted from the associated patent is shown in figure H-26, below (Reference 52: Lewis Counts, JoAnn Close, “Very Low Input Current JFET Amplifier," US Patent 4,639,683, filed February 7, 1986, issued January 27, 1987 (The ‘top-gate’ low-leakage JFET IC process and AD549 IC op amp)). In the AD549 circuit, the input FETs are J6 and J7 with the input signals applied to their top gates at 10 and 12. The back gates BG1 and BG2 of the pair are biased at approximately the same DC level by a bootstrap loop through Q14, and Q13-Q8. A second bootstrap loop through J4 and J8-J9 bootstraps the drains of J6-J7, thus providing for an input bias current level independent of CM voltage, over a ±10V range.
The AD549 electrometer IC op amp schematic (adapted from US Patent 4,639,683)
Figure 26: The AD549 electrometer IC op amp schematic (adapted from US Patent 4,639,683)

With this circuit, built on a junction-isolated process, the AD549L was able to achieve a bias current of less than 60fA, along with a 500μV(max) offset and a drift of 10μV/°C(max). It was provided in a hermetically sealed TO-99 package, with pin 8 connected to the case for guarding within the final application circuit. The AD549L sold for $15.45 in 100 piece lots.

In 1988 ADI introduced another electrometer amplifier based on the design of the AD549, the AD546 (Reference 53: "High-Performance Electrometer Op Amp in Plastic 8-Pin DIP ," Analog Dialogue, Vol. 23, No. 4, 1988, pp. 12 (The AD546 electrometer IC op amp)). JoAnn Close also designed this op amp, and it was offered in a plastic package with somewhat relaxed specifications (vis-à-vis the AD549). The AD546KN had a maximum bias current of 500fA, a maximum offset of 1mV, and a drift of 20μV/°C(typ). It sold for $4.50 in 100 piece lots.

The very latest electrometer amplifier in this series is the still supplied AD795. It is available in an SOIC package and has bias currents of 1pA or less (Reference 54: “Low-Noise, Low-Drift Precision Op Amps for Instrumentation,” Analog Dialogue, Vol. 27, No. 1, 1993 (The AD795 low noise, low IB and OP213 dual single-supply, precision IC op amps)).

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