LTC6602 datasheet(16/40 Pages) AD

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LTC6601-2

66012f

APPLICATIONS INFORMATION

FUNCTIONAL DESCRIPTION

The LTC6601 is designed to make the implementation of

high frequency fully-differential ﬁltering functions very

easy. A very low noise ampliﬁer is surrounded by 8 precision

matched resistors and 12 precision matched capacitors

so that a myriad of ﬁlter transfer functions limited only by

possible combinations and imagination can be conﬁgured

by hard wiring pins. The ampliﬁer itself is a wide band, low

noise and low distortion fully-differential ampliﬁer with ac-

curate output phase balancing. It is optimized for driving low

voltage, single-supply, differential input, analog-to-digital

converters (ADCs). The LTC6601’s outputs are capable

of swinging rail-to-rail on supplies as low as 2.7V, which

makes the ampliﬁer ideal for converting ground referenced,

single-ended signals into VOCM referenced differential

signals. Unlike traditional op amps which have a single

output, the LTC6601 has two outputs to process signals

differentially. This allows for two times the signal swing

in low voltage systems when compared to single-ended

output ampliﬁers. The balanced differential nature of the

ampliﬁer and matched surrounding components provide

even-order harmonic distortion cancellation, and less

susceptibility to common mode noise (like power supply

noise). The LTC6601 can be used as a single-ended input

to differential output ampliﬁer, or as a differential input to

differential output ampliﬁer.

Figure 3 shows the basic ﬁlter architecture. The Laplace

transfer function from VINDIFF to VOUTDIFF is given by the

following generalized equation for a 2nd order lowpass

ﬁlter:

VOUTDIFF

VINDIFF

Gain

2πfO •Q

+ s

2πfO

()2

Both Gain and Q of the ﬁlter are based on component ratios,

which match and track extremely well over temperature.

The corner frequency of the ﬁlter is a function of an RC

product. This RC product is trimmed to ±1% (typical) and

is not expected to drift by more than ±1% from nominal

over the entire temperature range –40°C to 85°C. As a

result, fully differential ﬁlters with tight magnitude, phase

tolerance and repeatability are achieved.

Although Figure 3 implies a differential input, the LTC6601

easily accepts single-ended inputs to either input, and will

faithfully replicate the signal at the output in differential

form.

The LTC6601’s output common mode voltage, deﬁned as

the average of the two output voltages, is independent of

the input common mode voltage, and is adjusted by apply-

ing a voltage on the VOCM pin. If the pin is left open, there

is an internal resistive voltage divider, which develops a

Figure 3. Basic Filter Topology and Equations

–

VOUT(DIFF)

66012 F03

VIN(DIFF)

fO =

R2 • R3 • C1• C2

•

1+ 1+ GAIN •

–

GAIN =

f3dB =

fO •

6089 •

3568 • Q4 1788 • Q2 + 447

()+1.287•105 •2•Q2 1

()

507.6 • Q

Q =

0.2236 • fO •

2.109 • 105 •

9.891• 1012 •f3dB

5.486 • 109 •fO

()+120• 5.526•109 •f

3dB

2 + 3.082 • 106 •fO2

()

16 • fO

2 • 8.29 • 109 •f3dB2 + 4.127 • 109 •fO2

() 6.638•1010 •f

3dB

()

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