Datasheet LT6600-20 (Analog Devices) - 9

LT6600-20
APPLICATIONS INFORMATION
20 The LT6600-20 was designed to process a variety of input 1dB PASSBAND GAIN 1MHz 25°C COMPRESSION POINTS signals including signals centered around the mid-sup- 0 1MHz 85°C ply voltage and signals that swing between ground and –20 3RD HARMONIC a positive voltage in a single supply system (Figure 1). 85°C –40 The range of allowable input common mode voltage (the 3RD HARMONIC 25°C average of V + – –60 IN and VIN in Figure 1) is determined by the power supply level and gain setting (see Distortion 2ND HARMONIC OUTPUT LEVEL (dBV) –80 25°C vs Input Common Mode Level in the Typical Performance 2ND HARMONIC –100 Characteristics section). 85°C –120 0 1 2 3 4 5 6 7
Common Mode DC Currents
1MHz INPUT LEVEL (VP-P) 66002 F06 In applications like Figure 1 and Figure 3 where the LT6600-20 not only provides lowpass fi ltering but also level
Figure 6. Output Level vs Input Level,
shifts the common mode voltage of the input signal, DC
Differential 1MHz Input, Gain = 1
currents will be generated through the DC path between place well before the output stage of the fi lter reaches the input and output terminals. Minimize these currents to supply rails, the input/output behavior of the IC shown decrease power dissipation and distortion. in Figure 6 is relatively independent of the power supply Consider the application in Figure 3. Pin 7 sets the output voltage. common mode voltage of the 1st differential amplifi er inside The two amplifi ers inside the LT6600-20 have indepen- the LT6600-20 (see the Block Diagram section) at 2.5V. dent control of their output common mode voltage (see Since the input common mode voltage is near 0V, there the Block Diagram section). The following guidelines will will be approximately a total of 2.5V drop across the series optimize the performance of the fi lter. combination of the internal 402Ω feedback resistor and the external 100Ω input resistor. The resulting 5mA common Pin 7 must be bypassed to an AC ground with a 0.01μF or mode DC current in each input path, must be absorbed by larger capacitor. Pin 7 can be driven from a low impedance the sources V + – IN and VIN . Pin 2 sets the common mode source, provided it remains at least 1.5V above V– and at output voltage of the 2nd differential amplifi er inside the least 1.5V below V+. An internal resistor divider sets the LT6600-20, and therefore sets the common mode output voltage of Pin 7. While the internal 11k resistors are well voltage of the fi lter. Since, in the example of Figure 3, Pin 2 matched, their absolute value can vary by ±20%. This differs from Pin 7 by 0.5V, an additional 2.5mA (1.25mA should be taken into consideration when connecting an per side) of DC current will fl ow in the resistors coupling external resistor network to alter the voltage of Pin 7. the 1st differential amplifi er output stage to fi lter output. Pin 2 can be shorted to Pin 7 for simplicity. If a different Thus, a total of 12.5mA is used to translate the common common mode output voltage is required, connect Pin 2 mode voltages. to a voltage source or resistor network. For 3V and 3.3V A simple modifi cation to Figure 3 will reduce the DC com- supplies the voltage at Pin 2 must be less than or equal to mon mode currents by 36%. If Pin 7 is shorted to Pin 2 the the mid supply level. For example, voltage (Pin 2)≤1.65V common mode output voltage of both op amp stages will on a single 3.3V supply. For power supply voltages higher be 2V and the resulting DC current will be 8mA. Of course, than 3.3V the voltage at Pin 2 should be within the voltage by AC coupling the inputs of Figure 3, the common mode of Pin 7 – 1V to the voltage of Pin 7 + 2V. Pin 2 is a high DC current can be reduced to 2.5mA. impedance input. 66002fb 9