Datasheet LTC6409 (Analog Devices) - 10

LTC6409
APPLICATIONS INFORMATION
RI R V F +IN that can be processed is even wider. The input common V–OUT + mode range at the op amp inputs depends on the circuit VINP configuration (gain), VOCM and VCM (refer to Figure 1). For – fully differential input applications, where VINP = –VINM, + the common mode input is approximately: VVOCM VOCM + – R R VCM V I F ICM = V+IN + V–IN ≈ VOCM • + VCM • – – 2 RI +RF RI +RF VINM R + I R V F –IN V 6409 F01 +OUT With single-ended inputs, there is an input signal compo- nent to the input common mode voltage. Applying only
Figure 1. Circuit for Common Mode Range
VINP (setting VINM to zero), the input common mode volt- age is approximately: single-ended signals to differential output signals for driving differential input ADCs. R V I ICM = V+IN + V–IN ≈ V + (2) 2 OCM • RI +RF
Output Common Mode and VOCM Pin
R V R V F INP F CM • + • The output common mode voltage is defined as the aver- R 2 R age of the two outputs: I + RF I + RF This means that if, for example, the input signal (VINP) VOUTCM = VOCM = V+OUT + V–OUT 2 is a sine, an attenuated version of that sine signal also appears at the op amp inputs. As the equation shows, the output common mode voltage is independent of the input common mode voltage, and
Input Impedance and Loading Effects
is instead determined by the voltage on the VOCM pin, by The low frequency input impedance looking into the VINP means of an internal common mode feedback loop. or VINM input of Figure 1 depends on how the inputs are If the VOCM pin is left open, an internal resistor divider driven. For fully differential input sources (VINP = –VINM), develops a default voltage of 1.25V with a 5V supply. The the input impedance seen at either input is simply: VOCM pin can be overdriven to another voltage if desired. RINP = RINM = RI For example, when driving an ADC, if the ADC makes a reference available for setting the common mode voltage, For single-ended inputs, because of the signal imbalance it can be directly tied to the V at the input, the input impedance actually increases over OCM pin, as long as the ADC is capable of driving the 40k input resistance presented the balanced differential case. The input impedance look- by the V ing into either input is: OCM pin. The Electrical Characteristics table speci- fies the valid range that can be applied to the VOCM pin (V RINP = RINM = RI OUTCMR). 1– 1 • RF 2 R
Input Common Mode Voltage Range
I + RF The LTC6409’s input common mode voltage (VICM) is Input signal sources with non-zero output impedances defined as the average of the two input pins, V+IN and can also cause feedback imbalance between the pair of V–IN. The valid range that can be used for VICM has been feedback networks. For the best performance, it is rec- specified in the Electrical Characteristics table (VICMR). ommended that the input source output impedance be However, due to external resistive divider action of the compensated. If input impedance matching is required gain and feedback resistors, the effective range of signals 6409fb 10 For more information www.linear.com/LTC6409 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Applications Related Parts Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Applications Related Parts