LTC1250

6

1250fb

–

+

1250 F02

LTC1250

Input Capacitance and Compensation

The large input transistors create a parasitic 55pF capaci-

react with the external feedback resistors to form a pole

which can affect amplifier stability. In low gain, high

impedance configurations, the pole can land below the

unity-gain frequency of the feedback network and degrade

phase margin, causing ringing, oscillation, and other

unpleasantness. This is true of any op amp, however, the

55pF capacitance at the LTC1250’s inputs can affect

Input Noise

The LTC1250, like all CMOS amplifiers, exhibits two types

of low frequency noise: thermal noise and 1/f noise. The

LTC1250 uses several design modifications to minimize

these noise sources. Thermal noise is minimized by rais-

high bias levels and using large transistor geometries. 1/f

noise is combated by optimizing the zero-drift nulling loop

to run at twice the 1/f corner frequency, allowing it to

reduce the inherently high CMOS 1/f noise to near thermal

levels at low frequencies. The resultant noise spectrum is

quite low at frequencies below the internal 5kHz clock

frequency, approaching the best bipolar op amps at 10Hz

and surpassing them below 1Hz (Figure 1). All this is

accomplished in an industry-standard pinout; the LTC1250

requires no external capacitors, no nulling or clock sig-

nals, and conforms to industry-standard 8-pin DIP and

8-pin SO packages.

S

APPLICATI

I FOR ATIO

stability with a feedback network impedance as low as

1.9k. This effect can be eliminated by adding a capacitor

across the feedback resistor, adding a zero which cancels

the input pole (Figure 2). The value of this capacitor should

be:

C

pF

A

F

V

≥

55

Figure 1. Voltage Noise vs Frequency

FREQUENCY (Hz)

20

30

40

50

80

0.01

1

LTC1250 F01

0

0.1

70

60

10

LTC1250

OP-07

OP-27

circuits, may actually increase low frequency noise. The

nulling circuitry in the LTC1250 closes a loop that includes

the external feedback network during part of its cycle. This

loop must settle to its final value within 150µs or it will not

fully cancel the 1/f noise spectrum and the low frequency

noise of the part will rise. If the loop is underdamped (large

offset will suffer.

from reaching a final value by the 150µs deadline. This

condition doesn’t affect the LTC1250’s offset or output

stability, but 1/f noise begins to rise. As a rule of thumb,

frequency at which the loop settles) for best 1/f perfor-

mance; values between 100pF and 500pF work well with

feedback resistors below 100k. This ensures adequate

gain at 7kHz for the LTC1250 to properly null. High value

feedback resistors (above 1M) may require experimenta-

tion to find the correct value because parasitics, both in the