LTC6261/LTC6262/LTC6263 typicAl ApplicAtions DRIVING A SARLTC6261 Driving LTC2362 ADC 0 The circuit next uses a traditional noninverting gain con- VIN = –1dBFS, 5kHz –10 f figuration to map a ground referenced input voltage signal S = 500kSps –20 SNR = 71.5dB to the full scale of a 500kS/s, 12 bit LTC2362 ADC. This THD = –83.6dB –30 SFDR = 86dB application takes advantage of the LTC6261 family’s com- –40 –50 bination of excellent common mode rejection, bandwidth, –60 supply current, and noise to enable high performance ADC –70 at low dissipation. The high bandwidth and open loop gain MAGNITUDE (dB) –80 combine to provide good distortion performance given the –90 –100 low supply current usage. The capacitor CF1 can be used –110 as needed to improve phase margin if there is any peak- –120 0 25 50 75 100 125 ing in the closed loop response due to total capacitance FREQUENCY (kHz) seen at the input terminals of the op amp as mounted on 626123 TA04a a PCB. The resistors should be chosen to minimize add- ing excessive noise while at the same time minimizing LTC6261 Driving LTC2362 ADC total current consumption and avoiding distortion due to 0 VIN = –1dBFS, 5kHz overloading the amplifier. The choice of resistor, then, will –10 fS = 250kSps –20 SNR = 72dB be commensurate with the input noise voltage and noise THD = –83.6dB –30 current of the LTC6261. Use of an output filter is critical SFDR = 86dB –40 in reducing noise and spurious high frequency content –50 that might alias. –60 –70 SAR ADC Driver MAGNITUDE (dB) –80 –90 CF1 –100 10pF –110 –120 0 10 20 30 40 50 60 RF1 FREQUENCY (kHz) 626123 TA04 1.74k 3.3V RF2 3.3V Results are shown with a 12 bit LTC2362 SAR ADC running 2.74k – V at both 500k Samples and 250k Samples. In both cases, U1 R DD VREF FILT CS 100Ω OUT SDO the ENOB is about 11.5. LTC6261 AIN LTC2362 + SCK IN CFILT 10nF OV GND DD ACTIVE FILTERS 626123 TA03 Second Order Bessel Filter Current consumption of the op amp circuit is 560µA at Ample bandwidth and low supply current allows deploy- 3.3V supply with the output centered at 1.65V. Increasing ment of active filters in portable and other low power the resistors with the same scaling factor will lower the applications. The second order Bessel filter provides a total consumption at the expense of more resistor noise. traditionally clean transient response. 626123fa 14 For more information www.linear.com/LTC6261 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information 5V Electrical Characteristics 1.8V Electrical Characteristics Typical Performance Characteristics Pin Functions Simplified Schematic Operation Applications Information Package Description Typical Application Related Parts
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