feature

AC performance; unity gain bandwidth: 40MHz; fast settling: 110 ns to 0.01%; slew rate: 300 V/s m; full power bandwidth: 4.7 MHz, 20 volts pp converted to a; 500 load five; DC performance; input bias Shift voltage: 1 mV max; Input voltage noise: 13 nV/Hz√ typical; Open loop gain: 45 V/mV input 1K load five; Output current: 50 mA min; Supply current: 12 mA max.

application

High-Speed Signal Conditioning; Video and Pulse Amplifiers; Data Acquisition Systems; Line Drivers; Active Filters; Available in 14-Pin Plastic Dip Sealed Cerdip, 12-Pin to 8-Pin Metal Can, and 20-Pin LCC Packages; Chip and MIL-STD-883B Available Components.

Product Description

The AD841 is a member of the wideband operational amplifier analog device family. This high-speed/precision family includes the AD840, which is stable at gains of 10 or greater, and the AD842, which is stable at gains of 2 or greater and has a minimum output current drive of 100 mA. The devices are fabricated using the junction-isolated complementary bipolar (CB) process for analog devices. This process allows for a combination of DC precision and broadband AC performance that was previously unachievable in monolithic op amps. In addition to the 40mhz unity gain bandwidth product, the AD841 has very fast settling characteristics, typically settling to within 0.01% of final value (10v steps) in 110ns.

Unlike many high frequency amplifiers, the AD841 does not require external compensation. It is stable over the entire operating temperature range. It also provides a low quiescent current of 12mA max, a minimum output current drive capability of 50mA, low input voltage noise of 13 nV/yHz and low input offset voltage of 1mV max.

The AD841's 300 V/µs slew rate and its 40 MHz gain bandwidth ensure excellent performance in video and pulse amplifier applications. This amplifier is ideal for high frequency signal conditioning circuits and wideband active filters. The extremely fast settling time of the AD841 makes it the first choice for data acquisition applications requiring 12-bit accuracy. The AD841 is also suitable for other applications such as high-speed DAC and ADC buffer amplifiers and other wideband circuits.

Application Highlights

1. The high slew rate and fast settling time of the AD841 make it ideal for DAC and ADC buffers and all types of video instrumentation circuits.

2. AD841 is a precision amplifier. It offers 0.01% or better accuracy and wideband performance previously only available in the mix.

3. The thermally balanced layout of the AD841 and the speed of the CB process allow the AD841 to settle to 0.01% in 110 ns without the long "tails" that occur with other fast op amps.

4. Laser wafer trimming reduces the input bias voltage to a maximum of 1 mV on K-class, thus eliminating the need for external bias zeroing in many applications. For added versatility, an offset zero pin is provided.

5. AD841 is an enhanced replacement for HA2541 .

AD841 – Typical Characteristics (at +258C and VS=615V unless otherwise noted)

offset zero

For high-speed op amps, the AD841 has a very low input bias voltage, but if an extra null is required, the circuit shown in Figure 21 can be used.

Enter Notes

An input resistor (RIN in Figure 20) is recommended in the circuit, and the input of the AD841 will be subjected to transient or continuous overload voltages that exceed the maximum differential limit of ±6 V. This resistor provides protection for the input transistor by limiting the maximum current that can be forced into the input.

For high performance circuits, resistors (RB in Figure 19 and Figure 20) are recommended to reduce bias current errors by matching the impedance of each input. If the bias current error is not eliminated, the output voltage error due to the bias current is more than an order of magnitude smaller than the error that exists.

AD841 Settling Time

Figures 22 and 24 show the stable performance of the AD841 in the test circuit shown in Figure 23.

Settling time is defined as the time interval from applying the ideal step function input until the closed loop amplifier output has been input and remains within a specified error band.

This definition includes the main components that make up the settling time. They include: (1) propagation delay through the amplifier; (2) slew time close to the final output value; (3) time to recover from slew-related overload; (4), linearity within a specified error band settlement.

In these terms, determining the time measurement is clearly a challenge and needs to be done accurately to ensure that the user considers the amplifier's application worthy of consideration.

The AD841's measurement of 0.01% settlement within 110 ns is accomplished by amplifying the error signal from the pseudo-summing junction using a high-speed dedicated hybrid error amplifier designed to test small settlement errors. The device under test is driving a 500Ω load. The input to the error amplifier is clamped to avoid possible problems associated with overdrive recovery of the oscilloscope input amplifier. The error amplifier gets an error of 10 from the false summing junction, and it contains a gain wiper to fine-tune the gain.

Figure 24 shows the “long-term” stability of the settling characteristics of the AD841 output after a 10V step. There is no sign of a settling tail after the initial transient recovery time. The use of a junction isolation process, coupled with careful layout, avoids these problems by minimizing the effects of transistor isolation capacitor discharge and thermally induced displacement of the circuit's operating point. These problems do not occur even under high output current conditions.

Ground and Bypass

When designing an actual circuit with the AD841, the user must remember that whenever high frequencies are involved, some special precautions must be taken. Circuits must use short interconnecting wires. Large ground planes should be used whenever possible to provide a low resistance, low inductance circuit path and minimize the effects of high frequency coupling. Sockets should be avoided because the increased inter-row capacitance reduces bandwidth.

The value of the feedback resistor should be low enough to ensure that the time constant formed by the circuit capacitance does not limit the performance of the amplifier. The recommended resistance value is less than 5 kΩ. If larger resistors must be used, a small (<10pf) feedback capacitor in parallel with the feedback resistor RF can be used to compensate for these stray capacitances and optimize the amplifier's dynamic performance in a specific application.

Power leads should be bypassed to ground as close to the amplifier pins as possible. A 2.2µF capacitor is recommended in parallel with a 0.1µF ceramic disc capacitor.

Capacitive load drive capability

Like all wideband amplifiers, the AD841 is sensitive to capacitive loading. The AD841 is designed to drive capacitive loads up to 20pF without degrading its rated performance. Capacitive loads greater than 20pf will degrade the dynamic performance of the part, but there should be no instability unless the load exceeds 100pf (for unity gain followers). A resistor in series with the output can be used to decouple larger capacitive loads.

Figure 25 shows a typical configuration for driving a large capacitive load. The 51Ω output resistor effectively isolates the high frequency feedback from the load and stabilizes the circuit. The low frequency feedback is returned to the amplifier summing junction through a low pass filter formed by the 51Ω resistor and load capacitor CL.

Use a radiator

The AD841 draws less quiescent power than most precision high-speed amplifiers and is specified to operate without a heat sink. However, when driving low impedance loads, the load current can be 4 to 5 times the quiescent current. This will cause a significant temperature rise. Performance can be improved by using a small heatsink such as Aavid Engineering #602B.

terminal line driver

The AD841 functions very well as a high-speed line driver for terminated or unterminated cables. Figure 26 shows the AD841 driving a double terminated cable in a follower configuration. The AD841 maintains a typical slew rate of 300V/µs, which means it can drive a ±10V, 4.7MHz signal or a ±3V, 15.9MHz signal.

The terminating resistance RT (when equal to the characteristic impedance of the cable) minimizes reflections at the far end of the cable. A rear end resistor (RBT, also equal to the characteristic impedance of the cable) can be placed between the AD841 output and the cable to suppress any spurious signals caused by the mismatch between the RT and the characteristic impedance of the cable. This will result in a "cleaner" signal, but since 1/2 the output voltage will drop through the CRBT, the op amp must provide double the output signal if there is no back end. Therefore, the full power bandwidth is cut in half.

If no termination is used, the cable will appear capacitively loaded. If this capacitive load is large, it should be separated from the AD841 by a resistor in series with the output (see above: Driving a capacitive load).

overdrive recovery

Figure 27 shows the overspeed recovery capability of the AD841.