LTC2066ISC6TRMPBF datasheet(15/30 Pages) AD

15 / 30 page

LTC2066/LTC2067/LTC2068

Rev. B

For more information www.analog.com

Using the LTC2066/LTC2067/LTC2068

The LTC2066/LTC2067/LTC2068 are single, dual, and

quad zero-drift operational amplifiers with the open-loop

voltage gain and bandwidth characteristics of a conven-

tional operational amplifier. Advanced circuit techniques

allow the LTC2066/LTC2067/LTC2068 to operate continu-

ously through its entire bandwidth while self-calibrating

unwanted errors.

Input Voltage Noise

Zero-drift amplifiers like the LTC2066/LTC2067/LTC2068

achieve low input offset voltage and 1/f noise by hetero-

dyning DC and flicker noise to higher frequencies. In early

zero-drift amplifiers, this process resulted in idle tones

at the self-calibration frequency, often referred to as the

chopping frequency. These artifacts made early zero-drift

amplifiers difficult to use. The advanced circuit techniques

used by the LTC2066/LTC2067/LTC2068 suppress these

spurious artifacts, allowing for trouble-free use.

Input Current Noise

For applications with high source and feedback imped-

ances, input current noise can be a significant contributor

to total output noise. For this reason, it is important to

consider noise current interaction with circuit elements

placed at the amplifier’s inputs.

VS = 5V

VCM = 2.5V

FREQUENCY (Hz)

0.1

100

10k 100k 1M

100

10k

2066 F01

Figure 1. Input Current Noise Spectrum

APPLICATIONS INFORMATION

The current noise spectrum of the LTC2066/LTC2067/

LTC2068 is shown in Figure 1. Low input current noise

is achieved through the use of MOSFET input devices and

self-calibration techniques to eliminate 1/f current noise.

As with all zero-drift amplifiers, there is an increase in

current noise at the offset-nulling frequency. This phe-

nomenon is discussed in the Input Bias Current and Clock

Feedthrough section.

Input current noise also rises with frequency due to

capacitive coupling of MOSFET channel thermal noise.

Input Bias Current and Clock Feedthrough

The input bias current of zero-drift amplifiers has differ-

ent characteristics than that of a traditional operational

amplifier. The specified input bias current is the DC aver-

age of transient currents which conduct due to the input

stage’s switching circuitry. In addition to this, junction

leakages can contribute additional input bias current at

elevated temperatures. Through careful design and the

use of an innovative boot-strap circuit, the input bias cur-

rent of the LTC2066/LTC2067/LTC2068 does not exceed

35pA at room and 150pA over the full temperature range.

This minimizes bias current induced errors even in high

impedance circuits.

Transient switching currents at the input interact with

source and feedback impedances, producing error volt-

ages which are indistinguishable from a valid input signal.

The resulting error voltages are amplified by the ampli-

fier’s closed-loop gain, which acts as a filter, attenuating

frequency components above the circuit bandwidth. This

phenomenon is known as clock feedthrough and is pres-

ent in all zero-drift amplifiers. Understanding the cause

and effect of clock feedthrough is important when using

zero-drift amplifiers.

For zero-drift amplifiers, clock feedthrough is proportional

to source and feedback impedances, as well as the mag-

nitude of the transient currents. These transient currents

have been minimized in the LTC2066/LTC2067/LTC2068

to allow use with high source and feedback impedances.

Many circuit designs require high feedback impedances

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