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LTC6261/LTC6262/LTC6263

626123fa

For more information www.linear.com/LTC6261

typicAl ApplicAtions

DRIVING A SAR

The circuit next uses a traditional noninverting gain con-

figuration to map a ground referenced input voltage signal

to the full scale of a 500kS/s, 12 bit LTC2362 ADC. This

application takes advantage of the LTC6261 family’s com-

bination of excellent common mode rejection, bandwidth,

supply current, and noise to enable high performance ADC

at low dissipation. The high bandwidth and open loop gain

combinetoprovidegooddistortionperformancegiventhe

low supply current usage. The capacitor CF1 can be used

as needed to improve phase margin if there is any peak-

ing in the closed loop response due to total capacitance

seen at the input terminals of the op amp as mounted on

a PCB. The resistors should be chosen to minimize add-

ing excessive noise while at the same time minimizing

total current consumption and avoiding distortion due to

overloading the amplifier. The choice of resistor, then, will

be commensurate with the input noise voltage and noise

current of the LTC6261. Use of an output filter is critical

in reducing noise and spurious high frequency content

that might alias.

LTC6261

3.3V

OUT

626123 TA03

RF1

1.74k

RF2

2.74k

CF1

10pF

RFILT

100

CFILT

10nF

3.3V

LTC2362

GND

SDO

SCK

OVDD

AIN

VDD

VREF

SAR ADC Driver

LTC6261 Driving LTC2362 ADC

Current consumption of the op amp circuit is 560µA at

3.3V supply with the output centered at 1.65V. Increasing

the resistors with the same scaling factor will lower the

total consumption at the expense of more resistor noise.

VIN = –1dBFS, 5kHz

fS = 500kSps

SNR = 71.5dB

THD = –83.6dB

SFDR = 86dB

FREQUENCY (kHz)

100

125

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

626123 TA04a

626123 TA04

VIN = –1dBFS, 5kHz

fS = 250kSps

SNR = 72dB

THD = –83.6dB

SFDR = 86dB

FREQUENCY (kHz)

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

Resultsareshownwitha12bitLTC2362SARADCrunning

at both 500k Samples and 250k Samples. In both cases,

the ENOB is about 11.5.

ACTIVE FILTERS

Second Order Bessel Filter

Ample bandwidth and low supply current allows deploy-

ment of active filters in portable and other low power

applications. The second order Bessel filter provides a

traditionally clean transient response.

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