LM6211 datasheet(11/19 Pages) NSC

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Application Notes

ADVANTAGES OF THE LM6211

High Supply Voltage, Low Power Operation

The LM6211 has performance guaranteed at supply volt-

ages of 5V and 24V. The LM6211 is guaranteed to be

operational at all supply voltages between 5V and 24V. In

this large range of operation, the LM6211 draws a fairly

constant supply current of 1 mA, while providing a wide

bandwidth of 20 MHz. The wide operating range makes the

LM6211 a versatile choice for a variety of applications rang-

ing from portable instrumentation to industrial control sys-

tems.

Low Input Referred Noise

The LM6211 has very low flatband input referred voltage

noise, 5.5 nV/

. The 1/f corner frequency, also very low,

is about 400 Hz. The CMOS input stage allows for an

extremely low input current (2 pA) and a very low input

referred current noise (0.01 pA/

). This allows the

LM6211 to maintain signal fidelity and makes it ideal for

audio, wireless or sensor based applications.

Low Input Bias Current and High Input Impedance

The LM6211 has a CMOS input stage, which allows it to

have very high input impedance, very small input bias cur-

rents (2 pA) and extremely low input referred current noise

(0.01 pA/

). This level of performance is essential for op

amps used in sensor applications, which deal with extremely

low currents of the order of a few nanoamperes. In this case,

the op amp is being driven by a sensor, which typically has a

source impedance of tens of M

Ω. This makes it essential for

the op amp to have a much higher impedance.

Low Input Capacitance

The LM6211 has a comparatively small input capacitance for

a high voltage CMOS design. Low input capacitance is very

beneficial in terms of driving large feedback resistors, re-

quired for higher closed loop gain. Usually, high voltage

CMOS input stages have a large input capacitance, which

when used in a typical gain configuration, interacts with the

feedback resistance to create an extra pole. The extra pole

causes gain-peaking and can compromise the stability of the

op amp. The LM6211 can, however, be used with larger

resistors due to its smaller input capacitance, and hence

provide more gain without compromising stability. This also

makes the LM6211 ideal for wideband transimpedance am-

plifiers, which require a wide bandwidth, low input referred

noise and low input capacitance.

RRO, Ground Sensing and Current Limiting

The LM6211 has a rail-to-rail output stage, which provides

the maximum possible output dynamic range. This is espe-

cially important for applications requiring a large output

swing, like wideband PLL synthesizers which need an active

loop filter to drive a wide frequency range VCO. The input

common mode range includes the negative supply rail which

allows direct sensing at ground in a single supply operation.

The LM6211 also has a short circuit protection circuit which

limits the output current to about 25 mA sourcing and 38 mA

sinking, and allows the LM6211 to drive short circuit loads

indefinitely. However, while driving short circuit loads care

should be taken to prevent the inputs from seeing more than

±0.3V differential voltage, which is the absolute maximum

differential input voltage.

Small Size

The small footprint of the LM6211 package saves space on

printed circuit boards, and enables the design of smaller and

more compact electronic products. Long traces between the

signal source and the op amp make the signal path suscep-

tible to noise. By using a physically smaller package, the

LM6211 can be placed closer to the signal source, reducing

noise pickup and enhancing signal integrity

STABILITY OF OP AMP CIRCUITS

Stability and Capacitive Loading

The LM6211 is designed to be unity gain stable for moderate

capacitive loads, around 100 pF. That is, if connected in a

unity gain buffer configuration, the LM6211 will resist oscil-

lation unless the capacitive load is higher than about 100 pF.

For higher capacitive loads, the phase margin of the op amp

reduces significantly and it tends to oscillate. This is because

an op amp cannot be designed to be stable for high capaci-

tive loads without either sacrificing bandwidth or supplying

higher current. Hence, for driving higher capacitive loads,

the LM6211 needs to be externally compensated.

An op amp, ideally, has a dominant pole close to DC, which

causes its gain to decay at the rate of 20 dB/decade with

respect to frequency. If this rate of decay, also known as the

rate of closure (ROC), remains at 20 dB/decade at the unity

gain bandwidth of the op amp, the op amp is stable. If,

however, a large capacitance is added to the output of the op

amp, it combines with the output impedance of the op amp to

create another pole in its frequency response before its unity

gain frequency (Figure 1). This increases the ROC to

40 dB/decade and causes instability.

In such a case a number of techniques can be used to

restore stability to the circuit. The idea behind all these

schemes is to modify the frequency response such that it

can be restored to a ROC of 20 dB/decade, which ensures

stability.

20120337

FIGURE 1. Gain vs. Frequency for an Op Amp

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