Datasheet LT6600-20 (Analog Devices) - 10

LT6600-20
APPLICATIONS INFORMATION Noise
2.5V 0.1μF The noise performance of the LT6600-20 can be evaluated SPECTRUM COILCRAFT with the circuit of Figure 7. ANALYZER R 3 V IN TTWB-1010 IN 1 INPUT – 25Ω 1:1 7 + 4 Given the low noise output of the LT6600-20 and the LT6600-20 2 50Ω 6dB attenuation of the transformer coupling network, it 8 – is necessary to measure the noise fl oor of the spectrum + 5 0.1μF 25Ω R 6 IN analyzer and subtract the instrument noise from the fi lter 66002 F07 noise measurement. –2.5V Example: With the IC removed and the 25Ω resistors
Figure 7
grounded, Figure 7, measure the total integrated noise (eS) of the spectrum analyzer from 10kHz to 20MHz. With the 50 250 IC inserted, the signal source (V ) VS = 5V IN) disconnected, and the Hz input resistors grounded, measure the total integrated noise /√ INTEGRA 40 200 RMS out of the fi lter (eO). With the signal source connected, set the frequency to 1MHz and adjust the amplitude until Y (nV TED NOISE (μ 30 150 V SPECTRAL DENSITY IN measures 100mVP-P. Measure the output amplitude, V 20 100 OUT, and compute the passband gain A = VOUT/VIN. Now V compute the input referred integratednoise (e RMS IN) as: ) 10 50 INTEGRATED (e NOISE SPECTRAL DENSIT e O )2 – (eS )2 IN = 0 0 A 0.1 1 10 100 Table 1 lists the typical input referred integrated noise for- FREQUENCY (MHz) 66002 F08 various values of RIN. Figure 8 is plot of the noise spectral
Figure 8. Input Referred Noise, Gain = 1
density as a function of frequency for an LT6600-20 with RIN = 402Ω using the fi xture of Figure 7 (the instrument
Power Dissipation
noise has been subtracted from the results). The LT6600-20 amplifi ers combine high speed with large-
Table 1. Noise Performance
signal currents in a small package. There is a need to
INPUT REFERRED
ensure that the die junction temperature does not exceed
PASSBAND INTEGRATED NOISE INPUT REFERRED
150°C. The LT6600-20 package has Pin 6 fused to the lead
GAIN (V/V) RIN 10kHz TO 20MHz NOISE dBm/Hz
frame to enhance thermal conduction when connecting to a 4 100Ω 42μVRMS –148 ground plane or a large metal trace. Metal trace and plated 2 200Ω 67μVRMS –143 through-holes can be used to spread the heat generated by 1 402Ω 118μVRMS –139 the device to the backside of the PC board. For example, The noise at each output is comprised of a differential on a 3/32" FR-4 board with 2oz copper, a total of 660 component and a common mode component. Using a square millimeters connected to Pin 6 of the LT6600-20 transformer or combiner to convert the differential outputs (330 square millimeters on each side of the PC board) to single-ended signal rejects the common mode noise and will result in a thermal resistance, θJA, of about 85°C/W. gives a true measure of the S/N achievable in the system. Without the extra metal trace connected to the V– pin to Conversely, if each output is measured individually and the provide a heat sink, the thermal resistance will be around noise power added together, the resulting calculated noise 105°C/W. Table 2 can be used as a guide when considering level will be higher than the true differential noise. thermal resistance. 66002fb 10