Datasheet

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LT6600-20

66002fb

TYPICAL APPLICATION

DESCRIPTION

Very Low Noise, Differential

Ampliﬁ er and 20MHz Lowpass Filter

The LT

6600-20 combines a fully differential ampliﬁ er with a

4th order 20MHz lowpass ﬁ lter approximating a Chebyshev

frequency response. Most differential ampliﬁ ers require

many precision external components to tailor gain and

bandwidth. In contrast, with the LT6600-20, two external

resistors program differential gain, and the ﬁ lter’s 20MHz

cutoff frequency and passband ripple are internally set.

The LT6600-20 also provides the necessary level shifting

to set its output common mode voltage to accommodate

the reference voltage requirements of A/Ds.

Using a proprietary internal architecture, the LT6600-20

integrates an antialiasing ﬁ lter and a differential ampli-

ﬁ er/driver without compromising distortion or low noise

performance. At unity gain the measured in band signal-

to-noise ratio is an impressive 76dB. At higher gains the

input referred noise decreases so the part can process

smaller input differential signals without signiﬁ cantly

degrading the output signal-to-noise ratio.

The LT6600-20 also features low voltage operation. The

differential design provides outstanding performance for

a 2V

P-P

signal level while the part operates with a single

3V supply.

The LT6600-20 is packaged in an SO-8 and is pin compat-

ible with stand alone differential ampliﬁ ers.

An 8192 Point FFT Spectrum

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

FEATURES

APPLICATIONS

Programmable Differential Gain via Two External

Resistors

Adjustable Output Common Mode Voltage

Operates and Speciﬁ ed with 3V, 5V, ±5V Supplies

0.5dB Ripple 4th Order Lowpass Filter with 20MHz

Cutoff

76dB S/N with 3V Supply and 2V

P-P

Output

Low Distortion, 2V

P-P

, 800Ω Load

2.5MHz: 83dBc 2nd, 88dBc 3rd

20MHz: 63dBc 2nd, 64dBc 3rd

Fully Differential Inputs and Outputs

SO-8 Package

Compatible with Popular Differential Ampliﬁ er

Pinouts

High Speed ADC Antialiasing and DAC Smoothing in

Networking or Cellular Base Station Applications

High Speed Test and Measurement Equipment

Medical Imaging

Drop-in Replacement for Differential Ampliﬁ ers

–

402

0.01µF

0.1µF

49.9

18pF

–

MID

OCM

–

OUT

LT6600-20

A/D

LTC1748

66002 TA01a

GAIN = 402/R

1µF

FREQUENCY (MHz)

AMPLITUDE (dB)

–60

–30

–20

66002 TA01b

–70

–80

–120

–100

–10

–40

–50

–90

–110

INPUT 5.9MHz

P-P

SAMPLE

= 80MHz

Summary of content (12 pages)

PAGE 1
LT6600-20 Very Low Noise, Differential Amplifier and 20MHz Lowpass Filter FEATURES DESCRIPTION n The LT®6600-20 combines a fully differential ampliﬁer with a 4th order 20MHz lowpass ﬁlter approximating a Chebyshev frequency response. Most differential ampliﬁers require many precision external components to tailor gain and bandwidth. In contrast, with the LT6600-20, two external resistors program differential gain, and the ﬁlter’s 20MHz cutoff frequency and passband ripple are internally set.
PAGE 2
LT6600-20 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW Total Supply Voltage .................................................11V Input Current (Note 8)..........................................±10mA Operating Temperature Range (Note 6).... –40°C to 85°C Speciﬁed Temperature Range (Note 7) .... –40°C to 85°C Junction Temperature ........................................... 150°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) ................
PAGE 3
LT6600-20 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. Unless otherwise speciﬁed VS = 5V (V+ = 5V, V– = 0V), RIN = 402Ω, and RLOAD = 1k. PARAMETER CONDITIONS MIN TYP MAX UNITS Filter Gain, RIN = 100 VIN = 0.5VP-P, fIN = DC to 260kHz, VS = 3V VIN = 0.5VP-P, fIN = DC to 260kHz, VS = 5V VIN = 0.5VP-P, fIN = DC to 260kHz, VS = 5V 11.6 11.5 11.4 12.1 12.0 11.9 12.6 12.5 12.
PAGE 4
LT6600-20 TYPICAL PERFORMANCE CHARACTERISTICS Passband Gain and Phase 0 0 –10 –2 –20 –4 –30 –6 –85 –8 –130 GAIN (dB) 2 –40 –50 VS = 5V GAIN = 1 TA = 25°C GAIN –40 PHASE –175 –12 –220 –70 –14 –265 –16 –310 VS = 5V GAIN = 1 TA = 25°C 1 10 FREQUENCY (MHz) –18 0.5 100 6.5 18.5 24.5 12.5 FREQUENCY (MHz) 66002 G01 GAIN (dB) 40 10 35 8 30 25 35 GROUP DELAY 6 4 30 25 0 15 10 –2 10 5 –4 5 –12 15 –14 –16 –6 0.5 0 30.5 18.5 24.5 12.5 FREQUENCY (MHz) 6.
PAGE 5
LT6600-20 TYPICAL PERFORMANCE CHARACTERISTICS Distortion vs Signal Level, VS = 3V –60 –70 3RD HARMONIC 1MHz INPUT –80 2ND HARMONIC 1MHz INPUT –90 2 3 INPUT LEVEL (VP-P) 5 4 –70 –80 VS = ±5V RL = 800Ω AT EACH OUTPUT GAIN = 1 TA = 25°C –100 0 1 2 3 INPUT LEVEL (VP-P) 4 66002 G09 –60 –70 –80 –90 500mVP-P 1MHz INPUT, GAIN = 4, RL = 800Ω AT EACH OUTPUT –100 –70 –80 –90 –100 5 –3 3 –2 –1 0 1 2 INPUT COMMON MODE VOLTAGE RELATIVE TO PIN 7 (V) 66002 G11 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS =
PAGE 6
LT6600-20 PIN FUNCTIONS IN– and IN+ (Pins 1, 8): Input Pins. Signals can be applied to either or both input pins through identical external resistors, RIN. The DC gain from differential inputs to the differential outputs is 402Ω/RIN. VOCM (Pin 2): Is the DC Common Mode Reference Voltagefor the 2nd Filter Stage. Its value programs the common mode voltage of the differential output of the ﬁlter.
PAGE 7
LT6600-20 APPLICATIONS INFORMATION Interfacing to the LT6600-20 output voltage is 1.65V, and the differential output voltage is 2VP-P for frequencies below 20MHz. The common mode output voltage is determined by the voltage at Pin 2. Since Pin 2 is shorted to Pin 7, the output common mode is the mid-supply voltage.
PAGE 8
LT6600-20 APPLICATIONS INFORMATION In Figure 3 the LT6600-20 is providing 12dB of gain. The gain resistor has an optional 62pF in parallel to improve the passband ﬂatness near 20MHz. The common mode output voltage is set to 2V. Use Figure 4 to determine the interface between the LT6600-20 and a current output DAC. The gain, or “transimpedance,” is deﬁned as A = VOUT/IIN.
PAGE 9
LT6600-20 APPLICATIONS INFORMATION 20 OUTPUT LEVEL (dBV) 0 1dB PASSBAND GAIN COMPRESSION POINTS The LT6600-20 was designed to process a variety of input signals including signals centered around the mid-supply voltage and signals that swing between ground and a positive voltage in a single supply system (Figure 1).
PAGE 10
LT6600-20 APPLICATIONS INFORMATION Noise 2.5V 0.1μF The noise performance of the LT6600-20 can be evaluated with the circuit of Figure 7. VIN RIN 1 7 Given the low noise output of the LT6600-20 and the 6dB attenuation of the transformer coupling network, it is necessary to measure the noise ﬂoor of the spectrum analyzer and subtract the instrument noise from the ﬁlter noise measurement. 2 8 RIN COILCRAFT TTWB-1010 25Ω 1:1 3 – + 4 SPECTRUM ANALYZER INPUT LT6600-20 50Ω – + 6 5 0.
PAGE 11
LT6600-20 APPLICATIONS INFORMATION For a given supply voltage, the worst-case power dissipation occurs when the differential input signal is maximum, the common mode currents are maximum (see the Applications Information section regarding common mode DC currents), the load impedance is small and the ambient temperature is maximum. To compute the junction temperature, measure the supply current under these worst-case conditions, estimate the thermal resistance from Table 2, then apply the equation for TJ.
PAGE 12
LT6600-20 TYPICAL APPLICATION A 5th Order, 20MHz Lowpass Filter V+ 0.1μF VIN– VIN+ C= R R 1 7 C 2 R 8 R 1 2π • R • 20MHz 3 – + 4 VOUT+ LT6600-20 – + 5 0.1μF 6 GAIN = 402Ω , MAXIMUM GAIN = 4 V– 2R Amplitude Response VOUT– 66002 TA02a Transient Response, Gain = 1 10 0 –10 GAIN (dB) –20 VOUT+ 50mV/DIV –30 –40 DIFFERENTIAL INPUT 200mV/DIV –50 –60 –70 –80 VS = ±2.5V GAIN = 1 C = 39pF R = 200Ω TA = 25°C –90 0.