Datasheet LT6600-15 (Analog Devices) - 9

LT6600-15
APPLICATIONS INFORMATION
20 The LT6600-15 was designed to process a variety of input 1dB COMPRESSION 25°C POINTS signals including signals centered around the mid-sup- 0 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 The range of allowable input common mode voltage (the –40 3RD HARMONIC average of V + – 25°C IN and VIN in Figure 1) is determined by –60 the power supply level and gain setting (see Distortion vs OUTPUT LEVEL (dBV) 2ND HARMONIC Input Common Mode Level in the Typical Performance –80 85°C Characteristics section). 2ND HARMONIC, 25°C –100 0 1 2 3 4 5 6 7
Common Mode DC Currents
1MHz INPUT LEVEL (VP-P) 660015 F06 In applications like Figure 1 and Figure 3 where the LT6600-15 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 supply rails, the input/output behavior of the IC shown input and output terminals. Minimize these currents to in Figure 6 is relatively independent of the power supply decrease power dissipation and distortion. voltage. Consider the application in Figure 3. Pin 7 sets the output The two amplifi ers inside the LT6600-15 have indepen- common mode voltage of the 1st differential amplifi er dent control of their output common mode voltage (see inside the LT6600-15 (see the Block Diagram section) at the Block Diagram section). The following guidelines will 2.5V. Since the input common mode voltage is near 0V, there optimize the performance of the fi lter. will be approximately a total of 2.5V drop across the series combination of the internal 536Ω feedback resistor and the Pin 7 must be bypassed to an AC ground with a 0.01μF or external 133Ω input resistor. The resulting 3.7mA common larger capacitor. Pin 7 can be driven from a low impedance mode DC current in each input path, must be absorbed by source, provided it remains at least 1.5V above V– and at the sources V + and V –. Pin 2 sets the common mode least 1.5V below V+. An internal resistor divider sets the IN IN output voltage of the 2nd differential amplifi er inside the voltage of Pin 7. While the internal 11k resistors are well LT6600-15, and therefore sets the common mode output matched, their absolute value can vary by ±20%. This voltage of the fi lter. Since, in the example of Figure 3, Pin 2 should be taken into consideration when connecting an differs from Pin 7 by 0.5V, an additional 2.5mA (1.25mA external resistor network to alter the voltage of Pin 7. per side) of DC current will fl ow in the resistors coupling Pin 2 can be shorted to Pin 7 for simplicity. If a different the 1st differential amplifi er output stage to fi lter output. common mode output voltage is required, connect Pin 2 Thus, a total of 9.9mA is used to translate the common to a voltage source or resistor network. For 3V and 3.3V mode voltages. supplies the voltage at Pin 2 must be less than or equal to A simple modifi cation to Figure 3 will reduce the DC com- the mid supply level. For example, voltage (Pin 2) ≤ 1.65V mon mode currents by 40%. If Pin 7 is shorted to Pin 2 on a single 3.3V supply. For power supply voltages higher the common mode output voltage of both op amp stages than 3.3V the voltage at Pin 2 should be within the voltage will be 2V and the resulting DC current will be 6mA. Of of Pin 7 – 1V to the voltage of Pin 7 + 2V. Pin 2 is a high course, by AC coupling the inputs of Figure 3, the common impedance input. mode DC current can be reduced to 2.5mA. 660015fb 9