“Differential drivers can be driven by single-ended or differential signals, and today we will analyze both cases using unterminated or terminated signal sources. Figure 1 shows a differential driver driven by a balanced unterminated signal source. This is usually the case for low-impedance signal sources, where the connection distance between the source and the driver is very short.
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Differential drivers can be driven by single-ended or differential signals, and today we will analyze both cases using unterminated or terminated signal sources.
01 Differential input, unterminated signal source
Figure 1 shows a differential driver driven by a balanced unterminated signal source. This is usually the case for low-impedance signal sources, where the connection distance between the source and the driver is very short.
Figure 1: Differential Input, Unterminated Signal Source
The design input is the source impedance RSGain setting resistor RG1and the desired gain G. Note: The gain is relative to the signal voltage source VSIGTake measurements.
with respect to the signal source VSIGthe total value of the gain setting resistors is equal to RG1+RS/2. In addition, RG2=RG1. Thus, the desired feedback resistor value (RF1=RF2) can be calculated by the following formula:
02 Differential input, termination signal source
In many cases, a differential drive source needs to drive twisted pairs, which must be terminated to their characteristic impedance in order to maintain high bandwidth and minimize reflections, as shown in Figure 2.
Figure 2. Differential Input, Terminated Signal Source
The design input is the source impedance RSGain setting resistor RG1and the desired gain G. Note: For the termination case, the gain is the differential voltage with respect to the termination resistor (VIN=VD+CVD-) to measure.
For balanced differential drive, the input impedance RINequal to 2RG1. Termination resistor RTChoose according to the following conditions: RT || RIN =RS,or
Thus, the desired feedback resistor value (RF1=RF2) can be calculated by the following formula:
03 Single-ended input, unterminated signal source
In many applications, differential amplifiers provide an efficient way to convert single-ended signals to differential signals. Figure 3 shows the case of an unterminated single-ended driver.
Figure 3. Single-ended input, unterminated signal source
The design input is the source impedance RSGain setting resistor RG1and the desired gain G. Note: The gain is relative to the signal voltage source VSIGTake measurements.
To prevent VOCMTo produce a bad offset voltage at the differential output, the net impedance seen by the two inputs of the differential amplifier must be equal. therefore,
In this way, the feedback resistor value can be calculated by:
04 Single-ended input, terminated signal source
Figure 4 shows a very common application where a single-ended signal source drives a coaxial cable; to minimize reflections and maintain high bandwidth, the coaxial cable must be properly terminated.
The design input is the source impedance RSGain setting resistor RG1and the desired gain G. Note: The gain is relative to the voltage of the termination resistor, VINTake measurements.
Figure 4. Single-ended input, terminated signal source
Know the desired gain G, gain setting resistor RG1and signal source resistance RS, calculate the initial value of the feedback resistor RF1A. The final value of this resistor will be slightly higher due to the need to increase RG2to match the input impedance, which will be calculated by the formula below. The calculation process is as follows:
Input voltage VINwith the signal source voltage VSIGHas the following relationship:
To calculate the final value of the feedback resistor, use the Thevenin equivalent circuit shown in Figure 5.
Figure 5. Thevenin Equivalent Input Circuit
The output voltage can be expressed as a function of the source voltage:
