LT6232 datasheet(17/26 Pages) AD

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LTC6246/LTC6247/LTC6248

Rev C

For more information www.analog.com

Figure 3. Single Supply 12-Bit ADC Driver

TYPICAL APPLICATIONS

12-Bit ADC Driver

Figure3showstheLTC6246drivinganLTC236612-bitA/D

converter. The low wideband noise of the LTC6246 main-

tains a 70dB SNR even without the use of an intermediate

antialiasing RC filter. On a single 3.3V supply with a 2.5V

reference, a full –1dBFS output can be obtained without

the amplifier transitioning between input regions, thus

minimizing crossover distortion. Figure 4 shows an FFT

obtained with a sampling rate of 2.2Msps and a 350kHz

input waveform. Spurious free dynamic range is a quite

handsome 82dB.

LTC6246

–

624678 F03

LTC2366

VREF

GND

VDD

3.3V 2.5V

SDO

SCK

OVDD

3.3V

VIN

AIN

499

10pF

Figure 4. 350kHz FFT Showing 82dB SFDR

FREQUENCY (kHz)

–10

–30

–50

–70

–20

–40

–60

–80

–90

–100

–110

400

800

200

600

624678 F04

1000

fIN = 350.195kHz

fSAMP = 2.2Msps

SFDR = 82dB

SNR = 70dB

1024 POINT FFT

0.1µF

LTC6246

–

624678 F05

LT6003

–

VOUT = VR + IPD • 1M

–3dB BW = 700kHz

ICC = 2.2mA

OUTPUT NOISE = 160µVRMS MEASURED ON A 1MHz BW

VOUT IS REFERRED TO VR

AT ZERO PHOTOCURRENT, VOUT = VR

1M, 1%

6.8nF

FILM

OR NPO

NXP

BF862

PD1

OSRAM

SFH213

IPD

20k

10k

10M

0.1pF

1µF

Low Noise Low Power DC-Accurate Single Supply

Photodiode Amplifier

Figure 5 shows the LTC6246 applied as a low power high

performance transimpedance amplifier for a photodiode.

A low noise JFET Q1 acts as a current buffer, with R2 and

R3 imposing a low frequency gain of approximately 1.

Transimpedance gain is set by feedback resistor R1 to

1MΩ. R4 and R5 set the LTC6246 inputs at 1V below

the 3V rail, with C3 reducing their noise contribution. By

feedback this 1V also appears across R2, setting the JFET

quiescent current at 1mA completely independent of its

pinchoff voltage and IDSS characteristics. It does this by

placing the JFETs 1mA VGS at the gate referenced to the

source, which is sitting 1V above ground. For this JFET,

that will typically be about 500mV, and this voltage is

imposed as a reverse voltage on the photodiode PD1. At

zero IPD photocurrent, the output sits at the same volt-

age and rises as photocurrent increases. As mentioned

before, R2 and R3 set the JFET gain to 1 at low frequency.

Figure 5. Low Noise Low Power DC Accurate

Single Supply Photodiode Amplifier

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