Datasheet LT6600-5 (Analog Devices) - 9

LT6600-5
APPLICATIONS INFORMATION
20 the power supply level and gain setting (see the Electrical 1dB PASSBAND GAIN 1MHz TA = 25°C COMPRESSION POINTS Characteristics section). 0 1MHz TA = 85°C 3RD HARMONIC –20 TA = 85°C
Common Mode DC Currents
3RD HARMONIC –40 TA = 25°C In applications like Figure 1 and Figure 3 where the LT6600-5 –60 not only provides lowpass fi ltering but also level shifts the common mode voltage of the input signal, DC currents OUTPUT LEVEL (dBV) –80 2ND HARMONIC will be generated through the DC path between input and TA = 85°C –100 2ND HARMONIC output terminals. Minimize these currents to decrease TA = 25°C –120 power dissipation and distortion. 0 1 2 3 4 5 6 7 1MHz INPUT LEVEL (VP-P) Consider the application in Figure 3. Pin 7 sets the output 66005 F06 common mode voltage of the 1st differential amplifi er in-
Figure 6
side the LT6600-5 (see the Block Diagram section) at 2.5V. The two amplifi ers inside the LT6600-5 have independent Since the input common mode voltage is near 0V, there control of their output common mode voltage (see the will be approximately a total of 2.5V drop across the series Block Diagram section). The following guidelines will combination of the internal 806Ω feedback resistor and the optimize the performance of the fi lter for single supply external 200Ω input resistor. The resulting 2.5mA common operation. mode DC current in each input path, must be absorbed by Pin 7 must be bypassed to an AC ground with a 0.01μF or the sources V + – IN and VIN . Pin 2 sets the common mode higher capacitor. Pin 7 can be driven from a low impedance output voltage of the 2nd differential amplifi er inside the source, provided it remains at least 1.5V above V– and at LT6600-5, and therefore sets the common mode output least 1.5V below V+. An internal resistor divider sets the voltage of the fi lter. Since in the example, Figure 3, Pin 2 voltage of Pin 7. While the internal 11k resistors are well differs from Pin 7 by 0.5V, an additional 1.25mA (0.625mA matched, their absolute value can vary by ±20%. This per side) of DC current will fl ow in the resistors coupling should be taken into consideration when connecting an the 1st differential amplifi er output stage to fi lter output. external resistor network to alter the voltage of Pin 7. Thus, a total of 6.25mA is used to translate the common mode voltages. Pin 2 can be shorted to Pin 7 for simplicity. If a different common mode output voltage is required, connect Pin 2 A simple modifi cation to Figure 3 will reduce the DC com- to a voltage source or resistor network. For 3V and 3.3V mon mode currents by 36%. If Pin 7 is shorted to Pin 2, the supplies the voltage at Pin 2 must be less than or equal to common mode output voltage of both op amp stages will the mid-supply level. For example, voltage (Pin 2) ≤1.65V be 2V and the resulting DC current will be 4mA. Of course, on a single 3.3V supply. For power supply voltages higher by AC coupling the inputs of Figure 3 and shorting Pin 7 than 3.3V the voltage at Pin 2 can be set above mid-supply. to Pin 2, the common mode DC current is eliminated. The voltage on Pin 2 should not be more than 1V below the voltage on Pin 7. The voltage on Pin 2 should not be
Noise
more than 2V above the voltage on Pin 7. Pin 2 is a high The noise performance of the LT6600-5 can be evaluated impedance input. with the circuit of Figure 7. The LT6600-5 was designed to process a variety of input Given the low noise output of the LT6600-5 and the 6dB signals including signals centered around the mid-sup- attenuation of the transformer coupling network, it will ply voltage and signals that swing between ground and be necessary to measure the noise fl oor of the spectrum a positive voltage in a single supply system (Figure 1). analyzer and subtract the instrument noise from the fi lter The range of allowable input common mode voltage (the noise measurement. average of V + – IN and VIN in Figure 1) is determined by 66005fb 9