Datasheet LT6600-2.5 (Analog Devices) - 10

LT6600-2.5
APPLICATIONS INFORMATION
VMID can be allowed to fl oat, but it must be bypassed to The LT6600-2.5 was designed to process a variety of input an AC ground with a 0.01μF capacitor or some instability signals including signals centered around the mid-sup- maybe observed. VMID can be driven from a low impedance ply voltage and signals that swing between ground and source, provided it remains at least 1.5V above V– and at a positive voltage in a single supply system (Figure 1). least 1.5V below V+. An internal resistor divider sets the The range of allowable input common mode voltage (the voltage of V + – MID. While the internal 11k resistors are well average of VIN and VIN in Figure 1) is determined by matched, their absolute value can vary by ±20%. This the power supply level and gain setting (see Electrical should be taken into consideration when connecting an Characteristics). external resistor network to alter the voltage of VMID.
Common Mode DC Currents
VOCM can be shorted to VMID for simplicity. If a different common mode output voltage is required, connect V In applications like Figure 1 and Figure 3 where the OCM to a voltage source or resistor network. For 3V and 3.3V LT6600-2.5 not only provides lowpass fi ltering but also supplies the voltage at V level shifts the common mode voltage of the input signal, OCM must be less than or equal to the mid-supply level. For example, voltage (V DC currents will be generated through the DC path between OCM) ≤ 1.65V on a single 3.3V supply. For power supply voltages input and output terminals. Minimize these currents to higher than 3.3V the voltage at V decrease power dissipation and distortion. OCM can be set above mid-supply, as shown in Table 1. The voltage on VOCM Consider the application in Figure 3. VMID sets the output should not exceed 1V below the voltage on VMID. VOCM common mode voltage of the 1st differential amplifi er is a high impedance input. inside the LT6600-2.5 (see the Block Diagram section) at
Table 1. Output Common Range for Various Supplies
2.5V. Since the input common mode voltage is near 0V, there will be approximately a total of 2.5V drop across
SUPPLY DIFFERENTIAL OUT OUTPUT COMMON MODE VOLTAGE VOLTAGE SWING RANGE FOR LOW DISTORTION
the series combination of the internal 1580Ω feedback 3V 4V resistor and the external 402Ω input resistor. The result- P-P 1.4V ≤ VOCM ≤ 1.6V 2V ing 1.25mA common mode DC current in each input P-P 1V ≤ VOCM ≤ 1.6V + – 1V path,must be absorbed by the sources V and V . P-P 0.75V ≤ VOCM ≤ 1.6V IN IN 5V 8V V P-P 2.4V ≤ VOCM ≤ 2.6V OCM sets the common mode output voltage of the 2nd 4V differential amplifi er inside the LT6600-2.5, and therefore P-P 1.5V ≤ VOCM ≤ 3.5V 2V sets the common mode output voltage of the fi lter. Since, P-P 1V ≤ VOCM ≤ 3.75V 1V in the example of Figure 3, V P-P 0.75V ≤ VOCM ≤ 3.75V OCM differs from VMID by 0.5V, an additional 625μA (312μA per side) of DC current will ±5V 9VP-P –2V ≤ VOCM ≤ 2V fl ow in the resistors coupling the 1st differential amplifi er 4VP-P –3.5V ≤ VOCM ≤ 3.5V output stage to fi lter output. Thus, a total of 3.125mA is 2VP-P –3.75V ≤ VOCM ≤ 3.75V used to translate the common mode voltages. 1VP-P –4.25V ≤ VOCM ≤ 3.75V NOTE: VOCM is set by the voltage at this RIN. The voltage at VOCM should A simple modifi cation to Figure 3 will reduce the DC not exceed 1V below the voltage at VMID. To achieve some of the output common mode currents by 36%. If V common mode ranges shown in the table, the voltage at V MID is shorted toVOCM MID must be set externally to a value below mid supply. the common mode output voltage of both op amp stages will be 2V and the resulting DC current will be 2mA. Of course, by AC-coupling the inputs of Figure 3, the common mode DC current can be reduced to 625μA. 660025fe 10