Application Note 109 March 2007 Interfacing RF I/Q Modulators with Popular D/A Converters Doug Stuetzle Introduction Linear Technology’s High Frequency Product lineup includes a variety of RF I/Q modulators. The purpose of this application note is to illustrate the circuits required to interface these modulators with several popular D/A converters. Such circuits typically are required to maximize the voltage transfer from the DAC to the baseband inputs of the modulator, as well as provide some reconstruction filtering. 0.52V. If the common mode DC voltage applied to the modulator’s baseband input terminals deviates from this value by more than about 75mV, the device may not meet the specifications listed in the data sheet, because source voltage affects the current consumed by the mixer core. The common mode voltage is approximately constant over temperature. • High impedance, lower common mode DC voltage (LT5571, LT5572) The baseband (I,Q) interface for the modulators falls into one of three categories: The baseband input circuit of these modulators presents a very high differential impedance (about 90k). The I, Q inputs do not generate an internal DC voltage, but require an external bias voltage of approximately 0.5V for proper operation. The data sheet for these parts shows changes in performance for applied voltages above and below this level. • High impedance, higher common mode DC voltage (LT®5558, LT5518) These modulators incorporate a baseband input circuit that presents a high differential impedance (about 2.9k). This circuit generates an internal DC bias voltage of approximately 2.06V; the common mode DC voltage applied to the baseband terminals must match this voltage to within 40mV for proper operation. This corresponds to a source/sink current at each baseband pin of 400µA. Note that this voltage will decrease by about 2.3 mV/°C with temperature. • Low impedance, lower common mode DC voltage (LT5568, LT5528) Simplified equivalent input circuits (I, I, Q or Q) for these three classes of modulators are shown in Figure 1. Most D/A converters have differential outputs and fall into one of two categories: • 0mA to 20mA source current, compliance voltage typically –1V to 1.25V (category 1) Examples of these include the following: Analog Devices AD9777, AD9779 Texas Instruments DAC2904, DAC5672, DAC5674 Maxim MAX5875, MAX5895 In this case, the modulator’s baseband input circuit presents a low differential impedance (about 100Ω), and generates an internal DC voltage of approximately LT5571, LT5572 BASEBAND INPUT PIN LT5528, LT5568 BASEBAND INPUT PIN 20Ω 23Ω + – CONTROLLED VOLTAGE SOURCE LT5518, LT5558 200Ω BASEBAND INPUT PIN 1.3k + – CONTROLLED VOLTAGE SOURCE AN109 F01 Figure 1. Modulator Baseband Equivalent Circuits (I, I, Q or Q) an109f AN109-1 Application Note 109 • 0mA to 20mA source current, compliance voltage 2.8V to 3.8V (category 2) Examples of these include the following: Texas Instruments DAC5686, DAC5687 There are, in general, three passive networks that are used to couple the output of the D/A converter to the baseband inputs of the I/Q modulator: • Direct coupling • Transformer coupling • Voltage shift network Coupling networks serve three purposes. First, as all of the D/A converters discussed here are current sources/sinks, the network must provide elements to convert this current to a voltage. Second, the output compliance range of the D/A converter does not always match that of the I/Q modulator. The network addresses this issue, with an eye toward maximizing the gain from DAC output to baseband input. Lastly, these networks usually include reconstruction filters to attenuate the sampling images from the DAC. These filters are typically 3rd or 5th order differential LC filters, designed to match the differential 2.2µH DIFFERENTIAL IMPEDANCE 200Ω 91pF 2.2µH 2.2µH 91pF 2.2µH DIFFERENTIAL 91pF IMPEDANCE 200Ω AN109 F02 Note that if the modulator is to be used in a power amplifier predistortion loop, the baseband bandwidth must be several times higher than the desired baseband frequency. This is because the predistorted baseband signal contains harmonics that are used to cancel the distortion of the power amplifier. All of the resistor values shown in the example circuits in this note should be 1% tolerance. Imbalances in these networks can give rise to degraded carrier and sidetone suppression, so the more closely the resistances match each other, the better. Specifically, inequalities in the DC voltages applied to the baseband pins of the modulator will affect the carrier suppression. For example, an otherwise perfect modulator will show a carrier suppression of –40dBc given a 10mV DC offset at any one baseband terminal. Also, if the AC amplitudes of the input signals at the baseband pins are not equal, this can affect the sidetone suppression; see Figure 3 for an example of these effects. If filters are used, unequal phase shifts among the I and Q ports will degrade the sidetone suppression as well. A phase offset of 1°, for example, will degrade an otherwise perfect sidetone suppression to –41.2dBc. 0 0 –30 –10 –10 –40 –50 –60 –70 –80 –90 –100 0 0.5 1 1.5 2 2.5 3 3.5 4 QUADRATURE ERROR (DEG) 4.5 5 AN109 F03a MODULATOR CARRIER LEVEL (dBc) –20 MODULATOR SIDETONE LEVEL (dBc) MODULATOR SIDETONE LEVEL (dBc) Figure 2. Differential Filter Example terminating impedances presented by the surrounding network. An example of a passive lowpass filter for a 10MHz baseband bandwidth WCDMA application appears in Figure 2. This filter is a Chebyshev 0.1 dB ripple design. It is designed for a 200Ω differential impedance, with a 10MHz passband frequency and a 40dB cutoff frequency of 21.9MHz. –20 –30 –40 –50 –60 –70 –80 –90 –100 0 1 2 3 4 5 6 7 8 AMPLITUDE IMBALANCE (%) 9 10 AN109 F03b –20 –30 –40 –50 –60 –70 –80 –90 –100 0 1 2 3 4 5 6 7 8 BASEBAND DC OFFSET (mV) 9 10 AN109 F03c Figure 3. Effects of Modulator Imperfections an109f AN109-2 Application Note 109 The simplest configuration is direct coupling between the output of the DAC and the input of the modulator; see Figure 4. In this case, the network consists of a shunt resistor to ground on each port. This type of network can be used with category 1 DAC’s, which have a typical output compliance range of –1V to 1.25V. The DAC is programmed to provide a midrange value of source current, which is typically 10mA. This current flowing into a 52Ω resistor yields a DC voltage of 0.52V. The resulting voltage fits well within the output compliance range of category 1 DAC’s, and matches the 0.52V required for proper operation of the LT5528, LT5568, LT5571 and the LT5572. The maximum output level of the modulator can be increased by increasing the current-to-voltage conversion ratio at the baseband inputs. One way to do this is to change the 52Ω shunt resistor to a higher value, and connect it to a negative voltage rail (instead of ground). This will maintain the DAC within its output compliance voltage range. An example appears in Figure 5. A 1% percent difference between the I+ and I– pins of the DAC amounts to a 50mV change in the DC voltage applied to the modulator baseband pins. This has a large effect on carrier suppression, as it will bias the mixer core asymmetrically. 0.52V DC ±0.24V AC MAX RF POUT = –2.4dBm 0.52V DC ±0.25V AC 0.5V DC ±0.5V AC MAX RF POUT = 7.4dBm 0.5V DC ±0.5V AC CATEGORY 1 DAC CATEGORY 1 DAC I+ 45Ω I+ 10mA DC ±10mA AC I+ 52.3Ω LPF 105Ω IN 90Ω OUT AC GND AT 0.52V DC 10mA DC ±10mA AC 10mA DC ±10mA AC 100Ω 100Ω 90k LPF 200Ω IN 200Ω OUT LT5528 LT5568 I– 45Ω I– I+ LT5571 LT5572 I– 52.3Ω AC GND AT 0.52V DC I– 10mA DC ±10mA AC 100Ω 100Ω AN109 F04 Figure 4. Category 1 DAC Passive DC-Coupled Interfaces 0.52V DC ±0.33V AC MAX RF POUT = 0.4dBm 0.52V DC ±0.5V AC CATEGORY 1 DAC 549Ω –5V 549Ω I– 10mA DC ±10mA AC 0.5V DC ±1.16V AC MAX RF POUT = 9.3dBm CATEGORY 1 DAC I+ 45Ω I+ 10mA DC ±10mA AC 0.5V DC ±0.5V AC 243Ω LPF 200Ω IN 90Ω OUT AC GND AT 0.52V DC I+ 10mA DC ±10mA AC 549Ω 200Ω LT5528 LT5568 LPF 200Ω IN 1100Ω OUT 90k –5V LT5571 LT5572 549Ω I– 45Ω AC GND AT 0.52V DC I+ I– I– 10mA DC ±10mA AC AN109 F05 Figure 5. Category 1 DAC Passive DC-Coupled Interfaces with Negative Bias Rail (Only I Channel Shown. Q Channel Interfaces are Identical) an109f AN109-3 Application Note 109 In some cases, the reconstruction filters shown in the example networks are designed with unequal termination impedances. This is done to maximize the voltage gain from the DAC output to the modulator input. In Figure 5 the network used to couple the category 1 DAC to the LT5528/LT5568 includes a lowpass filter with an input impedance of 200Ω and an output impedance of 90Ω. The net differential impedance presented to the DAC output is then 50Ω, while the output impedance of the filter matches the input impedance of the modulator (~90Ω). In some cases, the compliance range of the DAC will not include the DC input voltage of the modulator. One approach to this problem is transformer coupling. See Figure 6. The transformer effectively provides AC coupling between the DAC and the modulator. The output compliance voltage of the DAC is accommodated by connecting the center tap of the transformer to the appropriate voltage. One key issue with any network that provides an AC-coupled interface is the low frequency corner. In the case of available transformers, this corner may be as low as 4kHz. Nevertheless, the removal of low frequency baseband signal information may be a problem in some applications. Other cases may require that the carrier suppression of the modulator be optimized by adding small offsets to the DC voltages applied to the modulator inputs. This consideration rules out AC-coupled approaches, unless there is provision for auxiliary trim DAC’s at the modulator baseband pins. 2.06V DC ±1V AC MAX RF POUT = 8.5dBm CATEGORY 1 DAC 0.52V DC ±0.5V AC MAX RF POUT = 3.9dBm CATEGORY 1 DAC I+ I+ 1.5k 1:1 10mA DC ±10mA AC LPF 400Ω IN 400Ω OUT 400Ω AC GND AT 2.06V DC 400Ω I+ 10mA DC ±10mA AC LPF 400Ω IN 100Ω OUT 400Ω LT5518 LT5558 I– 1.5k I– 10mA DC ±10mA AC AC GND AT 2.06V DC 0V DC ±1V AC LT5528 LT5568 10mA DC ±10mA AC AC GND AT 2.06V DC 0V DC ±1V AC 0.52V DC ±0.25V AC MAX RF POUT = –2.1dBm CATEGORY 2 DAC I+ 10mA DC ±10mA AC AC GND AT 0.52V DC I– 45Ω I– 2.06V DC ±0.3V AC MAX RF POUT = 1.6dBm CATEGORY 2 DAC I+ 45Ω 1:1 I+ 1:1 3.3V 120Ω LPF 120Ω IN 120Ω OUT 120Ω 1.5k AC GND AT 2.06V DC I+ 10mA DC ±10mA AC 10mA DC ±10mA AC 3.3V DC ±0.3V AC 3.3V 100Ω LT5518 LT5558 I– 1.5k I– AC GND AT 2.06V DC I+ 45Ω 1:1 LPF 100Ω IN 100Ω OUT AC GND AT 0.52V DC LT5528 LT5568 I– 45Ω I– 10mA DC ±10mA AC 3.3V DC ±0.25V AC AC GND AT 0.52V DC AN109 F06 Figure 6. Transformer-Coupled Interfaces an109f AN109-4 Application Note 109 Another approach to the issue of compliance range is AC coupling via series capacitors. See Figure 7. Networks that use capacitors to implement AC coupling present a similar low-frequency issue as those that use transformers; the example shown in Figure 7 yields a corner frequency of 102Hz when a 1µF coupling capacitor is used. The compliance range of the DAC can also be matched to the DC requirement of the modulator with a resistive levelshifting network. See Figures 8 and 9. In some cases, the network is required to shift the DC voltage at the modulator input upward to accommodate the DAC. In other cases, the voltage shift required is downward. Resistive level-shifting networks are not frequency-sensitive, but they do come with a penalty. The resistive divider ratio inherent in these circuits attenuates the baseband signal significantly. For example, the network shown in Figure 8 connecting a category 2 DAC to an LT5518/LT5558 will attenuate the baseband signal by 4.2dB, assuming CATEGORY 1 DAC 1µF I+ 64.9Ω I– 64.9Ω CATEGORY 1 DAC AC GND AT 0.52V DC 1µF 2.06V DC ±0.48V AC MAX RF POUT = 5.7dBm 0.55V DC ±0.5V AC I+ 45Ω 215Ω LPF 130Ω IN 75Ω OUT 10mA DC ±10mA AC The coupling network between the DAC and the modulator can also include active elements, such as op amps and active filters. This approach enables a more accurate and balanced reconstruction filtering. If level shifting is required, this can often be incorporated into the active circuit. The LT1565-31, for example, is a 7th order linear phase lowpass filter with a corner frequency of 650kHz. It can be used to provide reconstruction filtering for single channel CDMA or RFID transmitters, and can accommodate the output compliance range of category 1 DACs. The LT1565-31 must be powered from ±5V rails in this 0.52V DC ±0.24V AC MAX RF POUT = –2.3dBm 0.5V DC ±0.32V AC 10mA DC ±10mA AC the bypass capacitors are not used. For the category 1 DAC example shown, the attenuation is 11dB. The bypass capacitors shown will eliminate this attenuation for all but the lowest frequencies. The network then becomes a lead-lag network, with pole and zero frequencies depending upon the resistor values required for the voltage transformation. 10mA DC ±10mA AC AC GND AT 0.52V DC I+ 1.5k 100Ω 100Ω AC GND AT 2.06V DC LPF 200Ω IN 190Ω OUT LT5528 LT5568 I– 45Ω 215Ω 1µF I+ 1µF I– 10mA DC ±10mA AC 100Ω LT5518 LT5558 I– 1.5k 100Ω AC GND AT 2.06V DC AN109 F07 CORNER AT 612Hz CORNER AT 102Hz Figure 7. AC-Coupled Interfaces an109f AN109-5 Application Note 109 0.5V DC ±0.5V AC 1µF 5V CATEGORY 1 DAC 5.62k 3.01k I+ 10mA DC ±10mA AC 100Ω 100Ω LPF 200Ω IN 185Ω OUT 1µF 3.01k I– 10mA DC ±10mA AC I+ 1.5k 100Ω AC GND AT 2.06V DC 5V 5.62k LT5518 LT5558 I– 1.5k 100Ω AC GND AT 2.06V DC 2.06V DC ±0.48V AC MAX RF POUT = 5.7dBm CORNER AT 80.9Hz LF ATTENUATION = 11dB CORNER AT 2.94kHz LF ATTENUATION = 4.2dB 5V CATEGORY 2 DAC 1µF 64.9Ω I+ 1.5k 75Ω I+ 10mA DC ±10mA AC 130Ω 1µF 5V 64.9Ω I– 10mA DC ±10mA AC 2.06V DC ±0.44V AC MAX RF POUT = 4.9dBm 2.06V DC ±0.32V AC AC GND AT 2.06V DC LPF 130Ω IN 240Ω OUT LT5518 LT5558 1.5k – I 75Ω 130Ω 3.3V DC ±0.32V AC AC GND AT 2.06V DC AN109 F08 Figure 8. Lead-Lag Coupling Interfaces to 2.1V Baseband Modulators an109f AN109-6 Application Note 109 CORNER AT 2.58kHz LF ATTENUATION = 16.4dB 5V CATEGORY 2 DAC 1µF 0.5V DC ±0.21V AC 64.9Ω 0.5V DC ±0.25V AC MAX RF POUT = 1.6dBm 174Ω I+ 10mA DC ±10mA AC I+ 100Ω 44.2Ω 64.9Ω I– 10mA DC ±10mA AC LPF 50Ω IN 200Ω OUT 1µF 5V 90k LT5571 LT5572 174Ω I– 100Ω 44.2Ω 3.3V DC ±0.13V AC CORNER AT 2.58kHz LF ATTENUATION = 20.4dB 5V CATEGORY 2 DAC 1µF 64.9Ω 10mA DC ±10mA AC 78.7Ω 51.1Ω 64.9Ω AC GND AT 0.52V DC LPF 55Ω IN 55Ω OUT 1µF 5V 10mA DC ±10mA AC I+ 45Ω 174Ω I+ I– 0.52V DC ±0.14V AC MAX RF POUT = –6.9dBm 0.52V DC ±0.14V AC LT5528 LT5568 45Ω – I 174Ω 51.1Ω 3.3V DC ±0.14V AC 78.7Ω AC GND AT 0.52V DC AN109 F09 Figure 9. Lead-Lag Coupling Interfaces to 0.5V Baseband Modulators an109f AN109-7 Application Note 109 output noise level of the modulator at this frequency, due to the noise of the LT1565-31, is then 84.2nV/√Hz. Given an RF system impedance of 50Ω, this is 45.5dB above kTB, or –128.5dBm/Hz. Note, however, that this elevation of noise level occurs only within the passband of the active filter. So the broadband output noise level of the modulator is not affected by the use of the active device. To assess the effect of the in-band noise, consider a 64QAM signal applied to the modulator. Assume, for example, the level of the modulated signal at the modulator output is 0dBm. The symbol-to-noise ratio is then 69.6dB, which will give rise to an EVM (error vector magnitude) of only 0.035%. This is a very small degradation in comparison to most system EVM specifications, which are on the order of 5% or more. case. It will drive the LT5518/LT5558 and LT5571/LT5572 modulators, and the output common mode voltage of the LT1565-31 is set by applying a DC voltage at Pin 3. See Figure 10 for details. Noise may also be an issue with the use of active devices in the baseband circuit. Any active device will have an output noise level that is higher than that of a passive circuit. Take Figure 10 showing the circuit coupling the category 1 DAC to the LT5518/LT5558 as an example. The noise output specification for the LT1565-31 is 118µVRMS over the bandwidth of the filter. Given a cutoff frequency of 650kHz, the effective noise bandwidth is approximately 780kHz. Assuming the noise level in band is flat, the equivalent output noise level of this device is 133.6nV/√Hz. The voltage gain of the LT5518 at 2GHz is –4dB, or a factor of 0.63. The CATEGORY 1 DAC I+ 10mA DC ±10mA AC LOWPASS CORNER AT 650kHz 0.5V DC ±0.5V AC 49.9Ω LT1565-31 POWERED FROM ±5V I+ 90k LT5571 LT5572 3 I– 10mA DC ±10mA AC I– 5V DC 49.9Ω 909Ω AN109 F10 100Ω 0.5V DC ±0.5V AC MAX RF POUT = 7.4dBm SETS OUTPUT COMMON MODE VOLTAGE AT 0.5V Figure 10. Active Filter Interface with 650kHz Bandwidth an109f AN109-8 Application Note 109 phase lowpass filter, and the gain can be configured with external resistors. The output common mode voltage is set by applying a DC voltage at Pin 3. See Figures 11 and 12 for details. The active device can also provide baseband gain in those cases where the DAC cannot drive the modulator to maximum signal swing. The LT6600-XX, for example, is an active lowpass filter available with bandwidths of 2.5MHz, 10MHz and 20 MHz. It includes a 4th-order linear 0.5V DC ±0.28V AC CATEGORY 1 DAC 113Ω I+ 10mA DC ±10mA AC 37.4Ω LT6600-X POWERED FROM 5V I+ 1.5k AC GND AT 2.06V DC LT5518 LT5558 3 113Ω I– 5V DC 37.4Ω 909Ω 10mA DC ±10mA AC 649Ω CATEGORY 1 DAC I+ 10mA DC ±10mA AC 0.5V DC ±0.44V AC 49.9Ω 10mA DC ±10mA AC 348Ω 2.06V DC ±1V AC MAX RF POUT = 8.5dBm SETS OUTPUT COMMON MODE VOLTAGE AT 2.06V I+ I+ 90k 10mA DC ±10mA AC 0.5V DC ±0.44V AC LT6600-X POWERED FROM 5V 348Ω 49.9Ω I– 10mA DC ±10mA AC 49.9Ω 909Ω LT5528 LT5568 3 I– 5V DC I+ 45Ω AC GND AT 0.52V DC LT5571 LT5572 3 I– AC GND AT 2.06V DC CATEGORY 1 DAC LT6600-X POWERED FROM 5V 348Ω I– 1.5k 348Ω I– 45Ω 5V DC 49.9Ω 866Ω AC GND AT 0.52V DC AN109 F11 100Ω 0.5V DC ±0.5V AC MAX RF POUT = 7.4dBm SETS OUTPUT COMMON MODE VOLTAGE AT 0.5V 100Ω 0.52V DC ±0.5V AC MAX RF POUT = 3.9dBm SETS OUTPUT COMMON MODE VOLTAGE AT 0.52V Figure 11. Category 1 DAC Active Filter Interfaces with 2.5MHz, 10MHz or 20MHz Bandwidth an109f AN109-9 Application Note 109 5V CATEGORY 2 DAC 64.9Ω 147Ω I+ 10mA DC ±10mA AC 3.3V DC ±0.37V AC 205Ω LT6600-X POWERED FROM 5V I+ 1.5k AC GND AT 2.06V DC 5V LT5518 LT5558 3 64.9Ω 147Ω I– 10mA DC ±10mA AC I– 1.5k 5V DC 205Ω 909Ω AC GND AT 2.06V DC 649Ω 2.06V DC ±1V AC MAX RF POUT = 8.5dBm SETS OUTPUT COMMON MODE VOLTAGE AT 2.06V 5V CATEGORY 2 DAC 64.9Ω 348Ω I+ 10mA DC ±10mA AC 5V 3.3V DC ±0.43V AC 205Ω CATEGORY 2 DAC LT6600-X POWERED FROM 5V I+ 64.9Ω 348Ω I– 10mA DC ±10mA AC 205Ω 10mA DC ±10mA AC 205Ω LT5528 LT5568 3 64.9Ω 348Ω I– 10mA DC ±10mA AC 909Ω I+ 45Ω 5V I– 5V DC LT6600-X POWERED FROM 5V AC GND AT 0.52V DC LT5572 LT5571 3 348Ω I+ 90k 5V 64.9Ω 3.3V DC ±0.43V AC 205Ω I– 45Ω 5V DC 866Ω AC GND AT 0.52V DC 100Ω 0.52V DC ±0.5V AC MAX RF POUT = 3.9dBm AN109 F12 100Ω 0.5V DC ±0.5V AC MAX RF POUT = 7.4dBm SETS OUTPUT COMMON MODE VOLTAGE AT 0.5V SETS OUTPUT COMMON MODE VOLTAGE AT 0.52V Figure 12. Category 2 DAC Active Filter Interfaces with 2.5MHz, 10MHz or 20MHz Bandwidth an109f AN109-10 Application Note 109 In the case of the LT5518/LT5558, the simple resistive divider used to set the output common mode level of the LT6600-XX does not allow this voltage to track the variation of the modulator baseband voltage over temperature. At temperature extremes, the common mode voltage applied to the baseband pins will be outside the optimum range for the LT5518/LT5558, and some performance degradation will occur. For best performance over temperature, the common mode voltage from the baseband drive circuit should closely match that at the modulator baseband terminals. An example of such a circuit appears in Figure 16 of the LT5558 data sheet. Op amps with differential outputs can also be used to level shift and amplify the baseband signals. The LT1994 is ideal for this purpose, as it can operate from a single 5V rail. See Figures 13 and 14 for details. 0.5V DC ±0.11V AC CATEGORY 1 DAC I+ 20Ω 174Ω I+ 1.5k LT1994 27.4Ω POWERED FROM 5V 10mA DC ±10mA AC AC GND AT 2.06V DC LT5518 LT5558 I– 10mA DC ±10mA AC 20Ω 27.4Ω 174Ω I– 1.5k 5V DC AC GND AT 2.06V DC 909Ω 2.06V DC ±1V AC MAX RF POUT = 8.5dBm 649Ω SETS OUTPUT COMMON MODE VOLTAGE AT 2.06V CATEGORY 1 DAC I+ 0.5V DC ±0.25V AC 49.9Ω 200Ω I+ LT1994 49.9Ω POWERED FROM 5V 10mA DC ±10mA AC 0.5V DC ±0.25V AC CATEGORY 1 DAC I+ 90k 200Ω 49.9Ω LT1994 49.9Ω POWERED FROM 5V 10mA DC ±10mA AC LT5571 LT5572 I– 10mA DC ±10mA AC 49.9Ω 49.9Ω 200Ω AC GND AT 0.52V DC LT5528 LT5568 I– 5V DC I+ 45Ω I– 10mA DC ±10mA AC 200Ω 49.9Ω 49.9Ω 909Ω I– 45Ω 5V DC 866Ω AC GND AT 0.52V DC AN109 F13 100Ω 0.5V DC ±1V AC MAX RF POUT = 9.3dBm 100Ω SETS OUTPUT COMMON MODE VOLTAGE AT 0.5V 0.52V DC ±1V AC MAX RF POUT = 7.9dBm SETS OUTPUT COMMON MODE VOLTAGE AT 0.52V Figure 13. Category 1 DAC Active Interfaces an109f Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. AN109-11 Application Note 109 3.3V DC ±0.3V AC 5V CATEGORY 2 DAC I+ 69.8Ω 232Ω 69.8Ω 10mA DC ±10mA AC I+ 1.5k LT1994 POWERED FROM 5V 232Ω AC GND AT 2.06V DC 5V I– LT5518 LT5558 69.8Ω 10mA DC ±10mA AC 232Ω 69.8Ω I– 1.5k 5V DC 232Ω AC GND AT 2.06V DC 909Ω 2.06V DC ±1V AC MAX RF POUT = 8.5dBm 649Ω SETS OUTPUT COMMON MODE VOLTAGE AT 2.06V 3.3V DC ±0.3V AC 5V CATEGORY 2 DAC I+ 64.9Ω 10mA DC ±10mA AC 5V CATEGORY 2 DAC 127Ω 75Ω LT1994 POWERED FROM 5V 205Ω 10mA DC ±10mA AC 127Ω 75Ω 205Ω LT1994 POWERED FROM 5V 5V I– I– 5V DC 127Ω 75Ω 205Ω LT5572 LT5571 64.9Ω 64.9Ω 10mA DC ±10mA AC 90k 5V I– I+ I+ 3.3V DC ±0.3V AC 10mA DC ±10mA AC 64.9Ω 909Ω AC GND AT 0.52V DC LT5528 LT5568 127Ω 75Ω 205Ω I+ 45Ω I– 45Ω 5V DC 866Ω AC GND AT 0.52V DC AN109 F14 100Ω 0.5V DC ±0.5V AC MAX RF POUT = 7.4dBm SETS OUTPUT COMMON MODE VOLTAGE AT 0.5V 100Ω 0.52V DC ±0.5V AC MAX RF POUT = 3.9dBm SETS OUTPUT COMMON MODE VOLTAGE AT 0.52V Figure 14. Category 2 DAC Active Interfaces an109f AN109-12 Linear Technology Corporation LT 0307 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2007