Single-Ended to Differential Amplifier Design Tips – Design Note 454 Philip Karantzalis and Tim Regan Introduction A fully differential amplifier is often used to convert a single-ended signal to a differential signal, a design which requires three significant considerations: the impedance of the single-ended source must match the single-ended impedance of the differential amplifier, the amplifier’s inputs must remain within the common mode voltage limits and the input signal must be level shifted to a signal that is centered at the desired output common mode voltage. In all cases, input impedance matching to the source impedance is necessary to prevent high frequency reflections. In designs where the single-ended source is DC coupled to a single supply differential amplifier, then level shifting and the common mode limits are also important considerations. The interaction of these three design parameters is non-trivial—component selection requires spreadsheet analysis using the equations described here. Input Impedance Matching If input AC coupling is used, then impedance matching is the only design issue. Figure 1 shows an example of a circuit matching a 50Ω single-ended source to an AC-coupled LTC ®6400-20 differential amplifier with a gain of 20dB set by internal resistors. The 66.5Ω resistor, RT, in parallel with the +IN input impedance, ZIN, matches the circuit input impedance to the 50Ω source. Differential balance is provided with the addition of the 28.7Ω resistor at the –IN input, R2. The balancing resistor assures equivalent feedback factors at the inputs, thus preventing large DC offsets. To calculate the external resistor values, start by calculating ZIN. Then calculate RT for impedance matching and the value of the R2 for differential balance. The overall single-ended to differential gain (GAIN) must take into account the input attenuation of the RS and RT resistive divider and the effect of adding R2. In this example, 11/08/454 ZIN RS 50Ω VIN R1 100Ω 0.1μF LTC6400-20 12.5Ω RF 1000Ω 13 +IN + – +OUT 8 50Ω RT 66.5Ω IN+ 14 +IN + – _ 15 –IN IN– + OUT– +OUTF 7 1.7pF 50Ω –OUTF 6 OUT+ RF 1000Ω R1 100Ω 0.1μF 12.5Ω 16 –IN R2 28.7Ω –OUT 5 VOUT– DN454 F01 RF2 • 4 • R12 RS2 8 • RF • R13 4 • R14 RF • 2 • R1 RS 2 • R1• R1 RS ZIN RT VOUT+ 2 • RF 2 • R1 RS RS • ZIN ZIN – RS GAIN R2 RS • R T RS R T – R1 R2 RF • R1• RS – R T RS • R T VOUT – VOUT VIN RS • R T • R1 R2 Figure 1. Impedance Matching for a Differential Amplifier with Fixed Gain Integrated Resistors the overall gain of the amplifier from signal source to differential output is only 4.44 even though the amplifier has a fixed gain of 10. By AC coupling at the input, the amplifier’s input common mode voltage is equal to its output common mode voltage and the single-ended signal is automatically level shifted to an output differential signal centered on the output common mode voltage. If the input common mode voltage is not 0V, and the source cannot deliver the DC current into 116.5Ω (50Ω + 66.5Ω), then it is also necessary to AC couple the 66.5Ω resistor. The DC Coupled Differential Amplifier A general purpose, DC coupled, single-ended-to-differential amplifier circuit with source impedance matching and input level shifting is shown in Figure 2. Level shifting is provided by the reference voltage (VREF ). If VREF is set to be equal to the input common mode voltage (VINCM) then the single-ended input signal is L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Figure 3 shows an example of a single-ended-to-differential amplifier matching a 75Ω source and level shifting from a 2.5V input common mode to a 1.25V output common mode voltage (typical level shifting required from a 5V single-ended circuit to a 3V differential circuit to drive a high speed ADC). The singleended-to-differential gain of the Figure 3 amplifier is 2 (the 1VP-P input signal is amplified into a 2VP-P differential output signal, a typical input voltage range of a high speed ADC). RF VINCM VINP-P VIN V+ RS VT + – R1 RT VOCM R2 VA VA R1 – + VOCM VOUTP-P + – VOCM VOUTP-P VREF GAIN = VREF = VINCM RF DN454 F02 2 • VOUTP-P = GN VINP-P GN2 • 4 • R12 RS2 2 • GN • R1• 2 • R1– RS R12 GN • 2 • R1 RS – R1 RF RT RS • R1 RF R1 RF – RS • GN 1 2 RS • R T R2 RS R T For linear operation, the amplifier’s input common mode limits must not be exceeded. Figure 2 shows the calculations for the bias voltage (VT) of the input R1 RF • RS RT • VINCM RS • RT • VOCM p VINP-P VT T-network (RS, RT and R1) and the common mode volt4 R1• RS R T RF • RS R T RS • R T age at the differential amplifier’s inputs. For example, in Figure 2. Impedance Matching and Level Shifting for a Figure 3, the 1.99V to 2.44V at the amplifier’s inputs (as Differential Amplifer with Gain Set By External Resistors calculated by the V equation) is well within the rail-to-rail A + shifted to a differential signal centered on the output input common mode range of the LTC6406 (0V to V ). common mode voltage (VOCM). Table 1. Sample of LTC High Speed Differential Amplifiers VA ¥ ´ GN RF • VINCM ¦ VOCM p • VINP-P µ • R1 R2 § ¶ 2 R1 R2 RF The design of a single-ended to differential amplifier with external resistors provides an additional design option: specifying the amplifier gain. Figure 2 shows the design equations when the RF and R1 resistors are selectable, not fixed. AMPLIFIER LTC6400-26 LTC6400-20 LTC6400-14 LTC6400-8 LTC6401-20 LTC6401-14 LTC6404-1 LTC6404-2 LTC6405 LTC6406 The design of this circuit begins with the value of R1. This resistor must be larger than the input source resistance but not so large as to increase circuit noise. Next, calculate the value of the feedback resistor RF using the desired gain (GN). Then calculate the value of resistors RT and R2. 2.5V VIN SLEW RATE V/μs 6670 4500 4800 3810 4500 3600 450 700 690 630 VOLTAGE NOISE nV/√Hz 1.5 2.1 2.5 3.7 2.1 2.5 1.5 1.5 1.6 1.6 GAIN V/V 20 10 5 2.5 10 5 R SET R SET R SET R SET 1.8pF 1VP-P + – GBW GHz 1.9 1.8 1.9 2.2 1.3 2 0.5 0.9 2.7 3 75Ω 150Ω 665Ω 3V 0.1μF 102Ω 1.25V VOCM – + + – ® 3V LT 6660-2.5 IN OUT GND 0.1μF 2.5V 0.1μF 43.2Ω 150Ω 10μF 1.25V 1VP-P 1.25V 1VP-P LTC6406 665Ω GAIN = 2 1.8pF DN454 F03 Figure 3. Putting it All Together: A 133MHz Differential Amplifier with External Gain Setting Resistors,Impedance Matching to a 75Ω Source and Shifting from 2.5V to 1.25V Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 3761 dn454fa LT/TP 1108 REV A 392K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008