Design Low Noise Differential Circuits Using the LT1567 Dual Amplifier Building Block Design Note 291 Philip Karantzalis Introduction Many communications systems use differential, low level (400mV to 1V peak-to-peak), analog baseband signals where the baseband circuitry operates from a single low voltage power supply (5V to 3V). Any differential ampliﬁer circuit used for baseband signal conditioning must have very low noise and an output voltage swing that includes most of the power supply range for maximum signal dynamic range. The LT®1567, a low noise operational ampliﬁer (1.4nV/√Hz voltage noise density) and a unity-gain inverter, is an excellent analog building block (see Figure 1) for designing low noise differential circuits. The gain bandwidth of the LT1567 ampliﬁer is 160MHz and its slew rate is sufﬁcient for signal frequencies up to 5MHz. The LT1567 operates from 2.7V to 12V total power supply. The output voltage swing is guaranteed to be 4.4V and 2.6V peak-to-peak, at 1k load with a single 5V and 3V power supply, respectively. The LT1567 is available in an 8-lead MSOP surface mount package. 1 tial output voltage noise density is 9.5 nV/√Hz and in a 4MHz noise bandwidth, the total differential output noise is 19μVRMS (with a low level 0.2VRMS differential signal, the signal-to-noise ratio is an excellent 80.4dB). The voltage on Pin 5 (VREF ) allows ﬂexible DC bias for the circuit and can be set by a voltage divider or a reference voltage source (with a single 3V power supply, the VREF range is 0.9V ≤ VREF ≤ 1.9V). In a single supply circuit, if the input signal is DC coupled, then an input DC voltage (VINDC) is required to bias the circuit within its linear region. If VINDC is within the VREF range, then VREF can be equal to VINDC and the output DC common mode voltage (VOUTCM) at VO1 and VO2 is equal to VREF. To maximize the unclipped LT1567 output swing however, the DC common mode output voltage must be set at V+/2. The input signal can be AC coupled to the circuit’s input resistor R1 and VREF also set to the DC common mode voltage required by any following circuitry (for example the input of an I and Q modulator). L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 6 600Ω 2 600Ω – R1 R2 – + 1 150Ω 600Ω 2 7pF 5 V+ 4 V– 6 7 + 3 8 VO1 VIN – + LT1567 DN191 F01 600Ω – 0.1μF Figure 1. LT1567 Analog Building Block 150Ω VREF A Single-Ended to Differential Ampliﬁer Figure 2 shows a circuit for generating a differential signal from a single-ended input. The differential output noise is a function of the noise of the ampliﬁers, the noise of resistors R1 and R2 and the noise bandwidth. For example, if R1 and R2 are each 200Ω, the differen08/02/291_conv 7 + 3 VO2 7pF 5 8 V+ V+ 4 0.1μF V– LT1567 DN191 F02 V R2 GAIN = O1 = VIN R1 VO1o("*/t7IN("*/ t7REF VO2 = –VO1t7REF VDIFF = VO2 – VO1 VDIFFt("*/t7IN – VREF) Figure 2. A Single-Ended Input to Differential Output Ampliﬁer A Differential Buffer/Driver Figure 3 shows an LT1567 connected as a differential buffer. The differential output voltage noise density is 7.7nV/√Hz. The differential buffer circuit of Figure 3, translates the input common mode DC voltage (VINCM) to an output common mode DC voltage (VOUTCM) set by the VREF voltage (VOUTCM = 2 • VREF – VINCM). For example, in a single 5V power supply circuit, if VINCM is 0.5V and VREF is 1.5V then VOUTCM is 2.5V. A Differential to Single-Ended Ampliﬁer Figure 4 shows a circuit for converting a differential input to a single-ended output. For a gain equal to one (R1 = R2 = 604Ω and VOUT = V2 – V1) the input referred differential voltage noise density is 9nV/√Hz and differential input signal-to-noise ratio is 80.9dB with 0.2VRMS input signal in a 4MHz noise bandwidth. The input AC common mode rejection depends on the matching of resistors R1 and R3 and the LT1567 inverter gain tolerance (common mode rejection is at least 40dB up to 1MHz with one percent resistors and two percent inverter gain tolerance). If the differential input is DC coupled, then VREF must be set equal to the input common mode voltage (VINCM). If VREF is greater than VINCM then a peak voltage on Pin 7 may exceed the output voltage swing limit. The DC voltage at the ampliﬁer’s output (VOUT, Pin 1) is VREF. LT1567 Free Design Software A spreadsheet-based design tool is available at www.linear.com for designing lowpass and bandpass ﬁlters using the LT1567. The simple-to-use spreadsheet requires the user to deﬁne the desired corner (or center) frequency, the passband gain and a capacitor value for a choice of second or third order Chebyshev or Butterworth lowpass or second order bandpass ﬁlters. The spreadsheet outputs the required external standard component values and provides a circuit diagram. Conclusion With one LT1567 and two or three resistors, it is easy to design low noise, differential circuits for signals up to 5MHz. The LT1567 can also be used to make low noise second and third order lowpass ﬁlters and second order bandpass ﬁlters with differential outputs. V2 R1 R2 C V1 VOUT 1 6 600Ω R3 = R1 2 600Ω – – + V2 604Ω 604Ω VO1 V1 0.1μF 1 VREF 5 V+ 600Ω 8 600Ω – 2 V+ – + 7 + 3 150Ω VO2 V+ V– LT1567 0.1μF 7pF R2 , R3 = R1 R1 VO = GAIN (V2 – V1) + VREF GAIN = f–3dB BANDWIDTH AT VOUT = 5 8 4 DN191 F04 0.1μF VREF 7pF 150Ω 6 7 + 3 V+ 4 0.1μF V– LT1567 DN191 F03 VO1 = –7t7REF VO2 = –7t7REF VDIFF = VO2 – VO1 = V1 – V2 OUTPUT DC COMMON MODE VOLTAGE, VOCMt7REF – VINCM Figure 3. A Differential Input and Output Buffer/Driver Data Sheet Download www.linear.com Linear Technology Corporation IF R1 = R3 = 604Ω, THEN 1 õ.)[ tπt3t$ R2 604Ω 1.21k 2.43k Vη* GAIN 9.0 8.4 8.1 NOISE AT VOUT("*/t7ηtðGηBW 1 2 4 fηBWtf –3dB *Vη IS THE INPUT REFERRED DIFFERENTIAL VOLTAGE NOISE %&/4*5:*/O7ð)[ Figure 4. A Differential Input-to-Single-Ended Output Ampliﬁer For applications help, call (408) 432-1900 dn291f_conv LT/TP 0802 351.5K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2002

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