7.2.1 Driving Capacitive Loads

Capacitive loads decrease the phase margin of all op amps. This is caused by the output resistance of the amplifier and the load capacitance forming an R-C phase lag network. This can lead to overshoot, ringing and oscillation. Slew rate limiting can also cause additional lag. Most op amps with a fixed maximum slew-rate will lag further and further behind when driving capacitive loads even though the differential input voltage raises. With the LM6132, the lag causes the slew rate to raise. The increased slew-rate keeps the output following the input much better. This effectively reduces phase lag. After the output has caught up with the input, the differential input voltage drops down and the amplifier settles rapidly.

Figure 30. Internal Block Diagram

These features allow the LM6132 to drive capacitive loads as large as 500 pF at unity gain and not oscillate. The scope photos (Figure 31 and Figure 32) show the LM6132 driving a 500 pF load. In Figure 31 , the lower trace is with no capacitive load and the upper trace is with a 500 pF load. Here we are operating on ±12V supplies with a 20 V_PP pulse. Excellent response is obtained with a C_f of 39 pF. In Figure 32, the supplies have been reduced to ±2.5V, the pulse is 4 V^PP and C_F is 39 pF. The best value for the compensation capacitor should be established after the board layout is finished because the value is dependent on board stray capacity, the value of the feedback resistor, the closed loop gain and, to some extent, the supply voltage.

Another effect that is common to all op amps is the phase shift caused by the feedback resistor and the input capacitance. This phase shift also reduces phase margin. This effect is taken care of at the same time as the effect of the capacitive load when the capacitor is placed across the feedback resistor.

The circuit shown in Figure 33 was used for Figure 31 and Figure 32.

Figure 31. Twenty-Volt Step Response:
with Cap Load (Top Trace)
without Cap Load (Bottom Trace)

Figure 32. Four-Volt Step Response:
with Cap Load (Top Trace)
without Cap Load (Bottom Trace)

Figure 33. Cap Load Test Circuit

Figure 34 shows a method for compensating for load capacitance (C_O) effects by adding both an isolation resistor R_O at the output and a feedback capacitor C_Fdirectly between the output and the inverting input pin. Feedback capacitor C_F compensates for the pole introduced by R_O and C_O, minimizing ringing in the output waveform while the feedback resistor R_F compensates for dc inaccuracies introduced by R_O. Depending on the size of the load capacitance, the value of R_Ois typically chosen to be between 100 Ω to 1 kΩ.

Figure 34. Capacitive Loading Compensation Technique

7.3.1 Three Op Amp Instrumentation Amp with Rail-to-Rail Input and Output

Using the LM6134, a 3 op amp instrumentation amplifier with rail-to-rail inputs and rail to rail output can be made. These features make these instrumentation amplifiers ideal for single supply systems.

Some manufacturers use a precision voltage divider array of 5 resistors to divide the common-mode voltage to get an input range of rail-to-rail or greater. The problem with this method is that it also divides the signal, so to even get unity gain, the amplifier must be run at high closed loop gains. This raises the noise and drift by the internal gain factor and lowers the input impedance. Any mismatch in these precision resistors reduces the CMR as well. Using the LM6134, all of these problems are eliminated.

In this example, amplifiers A and B act as buffers to the differential stage (Figure 35). These buffers assure that the input impedance is over 100 MΩ and they eliminate the requirement for precision matched resistors in the input stage. They also assure that the difference amp is driven from a voltage source. This is necessary to maintain the CMR set by the matching of R1–R2 with R3–R4.

Figure 35. Instrumentation Amplifier

7.3.2 Flat Panel Display Buffering

Three features of the LM6132/34 make it a superb choice for TFT LCD applications. First, its low current draw (360 μA per amplifier at 5 V) makes it an ideal choice for battery powered applications such as in laptop computers. Second, since the device operates down to 2.7 V, it is a natural choice for next generation 3V TFT panels. Last, but not least, the large capacitive drive capability of the LM6132 comes in very handy in driving highly capacitive loads that are characteristic of LCD display drivers.

The large capacitive drive capability of the LM6132/34 allows it to be used as buffers for the gamma correction reference voltage inputs of resistor-DAC type column (Source) drivers in TFT LCD panels. This amplifier is also useful for buffering only the center reference voltage input of Capacitor-DAC type column (Source) drivers such as the LMC750X series.

Since for VGA and SVGA displays, the buffered voltages must settle within approximately 4 μs, the well known technique of using a small isolation resistor in series with the amplifier's output very effectively dampens the ringing at the output.

With its wide supply voltage range of 2.7 V to 24 V, the LM6132/34 can be used for a diverse range of applications. The system designer is thus able to choose a single device type that serves many sub-circuits in the system, eliminating the need to specify multiple devices in the bill of materials. Along with its sister parts, the LM6142 and LM6152 that have the same wide supply voltage capability, choice of the LM6132 in a design eliminates the need to search for multiple sources for new designs.