General Instructions

The AD8541 /AD8542/AD8544 are single-, dual-, and quad-rail input and output, single-supply amplifiers with very low supply current and 1 MHz bandwidth. All supplies are guaranteed to operate from a single 2.7 V supply and a 5 V supply. These parts provide 1 MHz bandwidth with low current consumption of 45µA per amplifier.

The extremely low input bias current enables the AD8541/AD8542/AD8544 to be used in integrators, photodiode amplifiers, piezoelectric sensors, and other applications with high source impedance. Each amplifier supplies only 45µA of current, ideal for battery operation.

Rail-to-rail inputs and outputs are useful for designers to buffer ASICs in single-supply systems. The AD8541/AD8542/AD8544 are optimized to maintain high gain at lower supply voltages, making them useful for active filters and gain stages.

The AD8541/AD8542/AD8544 are specified over the extended industrial temperature range (–40°C to + 125 °C). The AD8541 is available in 5-wire SOT-23 , 5-wire SC70, and 8-wire SOIC packages. The AD8542 is available in 8-lead SOIC, 8-lead MSOP, and 8-lead TSSOP surface mount packages. The AD8544 is available in 14-lead narrow SOIC and 14-lead TSSOP SurfaceMount packages. All MSOP, SC70 and SOT versions are only available in tape and tape volumes.

theory of operation

AD854X Amplifier Considerations

The AD8541/AD8542/AD8544 amplifiers are general purpose operational amplifiers with improved performance. Compared to previous amplifiers, performance has been improved in several areas, including lower supply current with 1 MHz gain bandwidth, higher output current, and better performance at lower voltages.

Lower Supply Current with 1 MHz Gain Bandwidth

Each amplifier of the AD854X family typically uses 45 μA of current, which is far less than the 200 μA to 700 μA used in earlier parts with similar performance. This makes the AD854X family a good choice for upgrading portable designs to extend battery life. Alternatively, additional functionality and performance can be added at the same leakage current.

high output current

At a single 5V supply, the short-circuit current is typically 60µA. The AD854X amplifier can supply 30mA of output current, source or sink, even at 1V from the supply rail.

At lower voltages, both the source and sink are strong, 15 mA at 2.7 V and 18 mA at 3.0 V. For higher output currents, see the AD8531/AD8532/AD8534 section for 250mA of output current. obtained on.

Better performance at lower voltages, the AD854X family of parts are designed to provide better ac performance at 3.0V and 2.7V than previously offered parts. A typical gain-bandwidth product is close to 1MHz at 2.7V. The voltage gain at 2.7V and 3.0V is typically 500,000. Phase margin typically exceeds 60°C, making the part easy to use.

application

Notch filter

The AD854X has very high open-loop gain (especially at supply voltages below 4V), which makes it useful for all types of active filters. For example, Figure 36 illustrates the AD8542 in a classic dual-T notch filter design. The design of the double-T notch filter is simple, the output impedance is low, and the amount of operational amplifier used is small. In fact, this notch filter can be designed with just one op amp if no q tuning is required. Just remove U2 as shown in Figure 37. However, a major disadvantage of this circuit topology is ensuring that all rs and cs are closely matched. Components must be closely matched or the notch frequency shift and drift cause the circuit to no longer attenuate at the ideal notch frequency. To achieve the expected performance, a part tolerance of 1% or better or a special part screen is usually required. One way to reduce the sensitivity of the circuit to component mismatch is to increase r2 relative to r1, which decreases q. Lower q increases attenuation over a wider frequency range, but reduces attenuation at peak notch frequencies.

Figure 38 is an example of the ad8544 in a notch filter circuit. The critical matching requirement for a frequency-dependent negative-resistance notch is lower than for a double-T notch because the Q of the FDNR is proportional to a single resistor, R1. Although matching component values is still important, it is also much easier and/or cheaper to implement in fdnr circuits. For example, the twin-t notch filter uses three capacitors with two unique values, while the fdnr circuit uses only two capacitors with the same value. U3 is just a buffer to reduce the output impedance of the circuit.

Comparator function

The comparator function is a common application for alternate op amps in quaternary packages. Figure 39 illustrates a quarter of the AD8544 as a comparator in a standard overload detection application. Unlike many op amps, the AD854X family can double as a comparator because of its rail-to-rail differential input range, rail-to-rail output, and extremely high speed-to-power ratio. r2 is used to introduce lag. The ad854x has a 5µs propagation delay at 5v and a 5µs overload recovery time when used as a comparator.

Photodiode Applications

The AD854X family has very high impedance and the input bias current is typically around 4pa. This feature allows the AD854X op amps to be used in photodiode applications and other applications that require high input impedance. Note that the AD854X has a significant voltage offset that can be removed by capacitive coupling or software calibration.

Figure 40 illustrates a photodiode or current measurement application. The feedback resistor is limited to 10 MΩ to avoid excessive output offset. Furthermore, since the output offset related to the bias current is insignificant compared to the voltage offset contribution, there is no need to use a resistor on the unbiased input to cancel the bias current offset. For best performance, follow standard high-impedance layout techniques, which include: (1), shield the circuit; (2), clean the board; (3), place around the inverting input connected to the non-inverting input track; (4), use separate analog and digital power supply.