Datasheet LTC2066, LTC2067, LTC2068 (Analog Devices) - 15

link to page 15 link to page 15 link to page 15 LTC2066/LTC2067/LTC2068
APPLICATIONS INFORMATION Using the LTC2066/LTC2067/LTC2068
The current noise spectrum of the LTC2066/LTC2067/ The LTC2066/LTC2067/LTC2068 are single, dual, and LTC2068 is shown in Figure 1. Low input current noise quad zero-drift operational amplifiers with the open-loop is achieved through the use of MOSFET input devices and voltage gain and bandwidth characteristics of a conven- self-calibration techniques to eliminate 1/f current noise. tional operational amplifier. Advanced circuit techniques As with all zero-drift amplifiers, there is an increase in allow the LTC2066/LTC2067/LTC2068 to operate continu- current noise at the offset-nul ing frequency. This phe- ously through its entire bandwidth while self-calibrating nomenon is discussed in the Input Bias Current and Clock unwanted errors. Feedthrough section. Input current noise also rises with frequency due to
Input Voltage Noise
capacitive coupling of MOSFET channel thermal noise. Zero-drift amplifiers like the LTC2066/LTC2067/LTC2068 achieve low input offset voltage and 1/f noise by hetero-
Input Bias Current and Clock Feedthrough
dyning DC and flicker noise to higher frequencies. In early The input bias current of zero-drift amplifiers has differ- zero-drift amplifiers, this process resulted in idle tones ent characteristics than that of a traditional operational at the self-calibration frequency, often referred to as the amplifier. The specified input bias current is the DC aver- chopping frequency. These artifacts made early zero-drift age of transient currents which conduct due to the input amplifiers difficult to use. The advanced circuit techniques stage’s switching circuitry. In addition to this, junction used by the LTC2066/LTC2067/LTC2068 suppress these leakages can contribute additional input bias current at spurious artifacts, allowing for trouble-free use. elevated temperatures. Through careful design and the use of an innovative boot-strap circuit, the input bias cur-
Input Current Noise
rent of the LTC2066/LTC2067/LTC2068 does not exceed For applications with high source and feedback imped- 35pA at room and 150pA over the full temperature range. ances, input current noise can be a significant contributor This minimizes bias current induced errors even in high to total output noise. For this reason, it is important to impedance circuits. consider noise current interaction with circuit elements Transient switching currents at the input interact with placed at the amplifier’s inputs. source and feedback impedances, producing error volt- ages which are indistinguishable from a valid input signal. 10k VS = 5V The resulting error voltages are amplified by the ampli- V ) CM = 2.5V Hz fier’s closed-loop gain, which acts as a filter, attenuating frequency components above the circuit bandwidth. This Y (fA/√ 1k 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 100 zero-drift amplifiers. CURRENT NOISE DENSIT For zero-drift amplifiers, clock feedthrough is proportional to source and feedback impedances, as well as the mag- 100.1 1 10 100 1k 10k 100k 1M nitude of the transient currents. These transient currents FREQUENCY (Hz) 2066 F01 have been minimized in the LTC2066/LTC2067/LTC2068 to allow use with high source and feedback impedances.
Figure 1. Input Current Noise Spectrum
Many circuit designs require high feedback impedances Rev. B For more information www.analog.com 15 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Revision History Typical Application Related Parts