Datasheet LT6411 (Analog Devices) - 10

LT6411
APPLICATIONS INFORMATION Input Considerations
the blocking capacitor to the gain setting resistors sets The LT6411 input voltage range is from V the input and output DC common mode voltages equal. EE + 1V to V When using the LT6411 to drive an A-to-D converter, the CC – 1V. Therefore, on split supplies the LT6411 input range is always as large as or larger than the output swing. DC common mode voltage level will affect the harmonic On a single positive supply with a gain of +2 and IN– con- distortion of the combined amplifi er/ADC system. Figure 4 nected to ground, however, the input range limit of +1V shows the measured distortion of an LTC2249 ADC when limits the linear output low swing to 2V (1V multiplied by driven by the LT6411 at different common mode voltage the internal gain of 2). levels with the inputs confi gured as shown in Figure 3. Adjusting the DC bias voltage can optimize the design for The inputs can be driven beyond the point at which the the lowest possible distortion. output clips so long as input currents are limited to ±10mA. Continuing to drive the input beyond the output If the input signals are within the input voltage range limit can result in increased current drive and slightly and output swing of the LT6411, but outside the input increased swing, but will also increase supply current range of an ADC or other circuit the LT6411 is driving, and may result in delays in transient response at larger +V V+ levels of overdrive. IN+ CLARGE R1 LT6411 V OUT+ DC
DC Biasing Differential Amplifi er Applications
OV + VDC R2 – The inputs of the LT6411 must be DC biased within the input common mode voltage range, typically VEE + 1V to V+ V OUT– CC – 1V. If the inputs are AC coupled or DC biased be- – IN– CLARGE R1 yond the input voltage range of a driven A-to-D converter, VDC + VDC OV DC biasing or level shifting will be required. In the basic R2 6411 F03 circuit confi gurations shown in Figure 1, the DC input common mode voltage and the differential input signal
Figure 3. Using Resistor Dividers to Set the
are both multiplied by the amplifi er gain. In the gain of
Input Common Mode Voltage When AC Coupling
+2 confi guration, the DC common mode voltage gain can be set to unity by adding a capacitor at the IN– pins as shown in Figure 2. –50 VCC = 5V, VEE = 0V If the inputs are AC coupled or the LT6411 is preceded –55 AV= 2 TA = 25°C by a highpass fi lter, the input common mode voltage can –60 be set by resistor dividers as shown in Figure 3. Adding –65 HD3 +V –70 IN+ –75 LT6411 OUT+ DISTORTION (dBc) IM3 VDC + –80 VDC – –85 HD2 C –90 LARGE 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 – OUT– V IN– CM (V) + VDC 6411 F04 VDC
Figure 4. Harmonic and Intermodulation Distortion of the
6411 F02
LT6411 Driving an LTC2249 Versus DC Common Mode Voltage. Harmonic Distortion Measured with a –1dBFS Signal Figure 2. LT6411 Confi gured with a Differential Gain of 2 at 30.2MHz. Intermodulation Distortion Measured with Two and Unity DC Common Mode Gain –7dBFS Tones at 30.2MHz and 29.2MHz
6411f 10