LTC1668

Final Electrical Specifications LTC1668 16-Bit, 50Msps DAC February 2000 FEATURES s s s s s s s s s DESCRIPTIO 50Msps Update Rate 16-Bit Resolution High Spectral Purity: 87dB SFDR at 1MHz fOUT Differential Current Outputs 30ns Settling Time 5pV-s Glitch Impulse Low Power: 180mW from ± 5V Supplies TTL/CMOS (3.3V or 5V) Inputs Small Package: 28-Pin SSOP The LTC®1668 is a 16-bit, 50Msps differential current output DAC implemented on a high performance BiCMOS process with laser trimmed, thin-film resistors. The combination of a novel current-steering architecture and a high performance process produces a DAC with exceptional AC and DC performance. This is the first 16-bit DAC in the marketplace to exhibit an SFDR (spurious free dynamic range) of 87dB for an output signal frequency of 1MHz. Operating from ± 5V supplies, the LTC1668 can be configured to provide full-scale output currents up to 10mA. The differential current outputs of the DAC allow singleended or true differential operation. The –1V to 1V output compliance of the LTC1668 allows the outputs to be connected directly to external resistors to produce a differential output voltage without degrading the converter’s linearity. Alternatively, the outputs can be connected to the summing junction of a high speed operational amplifier, or to a transformer. The LTC1668 is available in a 28-pin SSOP and is fully specified over the industrial temperature range. , LTC and LT are registered trademarks of Linear Technology Corporation. APPLICATIO S s s s s s s s s Cellular Base Stations Multicarrier Base Stations Wireless Communication Direct Digital Synthesis (DDS) xDSL Modems Arbitrary Waveform Generation Automated Test Equipment Instrumentation TYPICAL APPLICATIO 16-Bit, 50Msps DAC 5V 0.1µF VDD LTC1668 REFOUT 0.1µF RSET 2k IREFIN 2.5V REFERENCE SIGNAL AMPLITUDE (dBm) 52.3Ω + – COMP1 C1 0.1µF COMP2 C2 0.1µF VSS AGND DGND CLK IOUT A 16-BIT HIGH SPEED DAC 52.3Ω IOUT B + VOUT 1VP-P DIFFERENTIAL – LADCOM DB15 DB0 1668 TA01 0.1µF – 5V CLOCK 16-BIT DATA INPUT INPUT 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. U Single Tone SFDR –5 –15 –25 –35 –45 –55 –65 –75 –85 –95 –105 0.05 6.3 FREQUENCY (1.25MHz/DIV) 12.55 1668 G01 U U fCLOCK = 25Msps fOUT = 1.007MHz AMPLITUDE = 0dBFS = –8.5dBm SFDR = 86dBc 1 LTC1668 ABSOLUTE (Note 1) AXI U RATI GS PACKAGE/ORDER I FOR ATIO TOP VIEW DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 1 2 3 4 5 6 7 8 9 28 DB14 27 DB15 (MSB) 26 CLK 25 VDD 24 DGND 23 VSS 22 COMP2 21 COMP1 20 IOUT A 19 IOUT B 18 LADCOM 17 AGND 16 IREFIN 15 REFOUT Supply Voltage (VDD) ................................................ 6V Negative Supply Voltage (VSS) ............................... – 6V Total Supply Voltage (VDD to VSS) .......................... 12V Digital Input Voltage .................... – 0.3V to (VDD + 0.3V) Analog Output Voltage (IOUT A and IOUT B) ........ (VSS – 0.3V) to (VDD + 0.3V) Power Dissipation ............................................. 500mW Operating Temperature Range LTC1668C .............................................. 0°C to 70°C LTC1668I ........................................... – 40°C to 85°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C ORDER PART NUMBER LTC1668CG LTC1668IG DB4 10 DB3 11 DB2 12 DB1 13 DB0 (LSB) 14 G PACKAGE 28-LEAD PLASTIC SSOP TJMAX = 110°C, θJA = 100°C/W Consult factory for Military grade parts. The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDD = 5V, VSS = – 5V, LADCOM = AGND = DGND = 0V, IOUTFS = 10mA. SYMBOL PARAMETER CONDITIONS DC Accuracy (Measured at IOUTA, Driving a Virtual Ground) Resolution Monotonicity INL Integral Nonlinearity DNL Differential Nonlinearity Offset Error Offset Error Drift GE Gain Error Internal Reference, RIREFIN = 2k External Reference, VREF = 2.5V, RIREFIN = 2k Gain Error Drift Internal Reference External Reference PSRR Power Supply Rejection Ratio VDD = 5V ±5% VSS = – 5V ± 5% Analog Output IOUTFS Full-Scale Output Current Output Compliance Range IFS = 10mA Output Resistance; RIOUTA, RIOUTB IOUTA, B to LADCOM Output Capacitance Reference Output Reference Voltage REFOUT Tied to IREFIN Through 2kΩ Reference Output Drift Reference Output Load Regulation ILOAD = 0mA to 5mA MIN 16 14 ±1 0.1 5 ±8 ±4 ±0.2 2 1 75 50 ±0.1 ±0.1 q q q ELECTRICAL CHARACTERISTICS TYP MAX UNITS Bits Bits LSB LSB % FSR ppm/°C % FSR % FSR ppm/°C ppm/°C % FSR/V % FSR/V mA V kΩ pF V ppm/°C mV/mA 1 –1 0.7 1.1 5 2.5 25 6 10 1 1.5 2.475 2.525 2 U W U U WW W LTC1668 The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDD = 5V, VSS = – 5V, LADCOM = AGND = DGND = 0V, IOUTFS = 10mA. SYMBOL PARAMETER Reference Input Reference Small-Signal Bandwidth Power Supply VDD VSS IDD ISS PDIS Positive Supply Voltage Negative Supply Voltage Positive Supply Current Negative Supply Current Power Dissipation IFS = 10mA, fCLK = 25Msps, fOUT = 1MHz IFS = 10mA, fCLK = 25Msps, fOUT = 1MHz IFS = 10mA, fCLK = 25Msps, fOUT = 1MHz IFS = 1mA, fCLK = 25Msps, fOUT = 1MHz q q q q q q ELECTRICAL CHARACTERISTICS CONDITIONS IFS = 10mA, CCOMP1 = 0.1µF MIN TYP 20 MAX UNITS kHz 4.75 – 4.75 5 –5 3 33 180 85 75 30 8 15 5 4 4 5.25 – 5.25 5 40 V V mA mA mW mW Msps ns ns pV-s pV-s ns ns pA/√Hz pA/√Hz Dynamic Performance (Differential Transformer Coupled Output, 50Ω Double Terminated, Unless Otherwise Noted) fCLOCK Maximum Update Rate q 50 tS tPD Output Settling Time Output Propagation Delay Glitch Impulse tr tf iNO AC Linearity SFDR Output Rise Time Output Fall Time Output Noise IFS = 10mA IFS = 1mA fCLK = 25Msps, fOUT = 1MHz 0dB FS Output – 6dB FS Output –12dB FS Output –18dB FS Output fCLK = 50Msps, fOUT = 1MHz fCLK = 50Msps, fOUT = 2.5MHz fCLK = 50Msps, fOUT = 5MHz fCLK = 50Msps, fOUT = 20MHz Spurious Free Dynamic Range Within a Window THD Digital Inputs VIH VIL IIN CIN tDS tDH tCLKH tCLKL Total Harmonic Distortion fCLK = 25Msps, fOUT = 1MHz, 2MHz Span fCLK = 50Msps, fOUT = 5MHz, 4MHz Span fCLK = 25Msps, fOUT = 1MHz fCLK = 50Msps, fOUT = 5MHz q q q To 0.1% FSR Single Ended Differential 50 30 Spurious Free Dynamic Range to Nyquist 78 87 87 86 80 84 80 77 65 dB dB dB dB dB dB dB dB dB dB –77 dB dB V V µA pF ns ns ns ns 86 96 88 –84 –76 Digital High Input Voltage Digital Low Input Voltage Digital Input Current Digital Input Capacitance Input Setup Time Input Hold Time Clock High Time Clock Low Time 2.4 0.8 ± 10 5 8 4 5 8 q q q q Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. 3 LTC1668 TYPICAL PERFOR A CE CHARACTERISTICS Single Tone SFDR –5 –15 SIGNAL AMPLITUDE (dBm) SIGNAL AMPLITUDE (dBm) –25 –35 –45 –55 –65 –75 –85 –95 –105 0.05 6.3 FREQUENCY (1.25MHz/DIV) Integral Nonlinearity 5 DIFFERENTIAL NONLINEARITY (LSB) 4 INTEGRAL NONLINEARITY (LSB) 3 2 1 0 –1 –2 –3 –4 –5 49152 32768 16384 DIGITAL INPUT CODE 65535 1668 G03 PI FU CTIO S REFOUT (Pin 15): Internal Reference Voltage Output. Nominal value is 2.5V. Requires a 0.1µF bypass capacitor to AGND. IREFIN (Pin 16): Reference Input Current. Nominal value is 1.25mA for IFS = 10mA. IFS = IREFIN • 8. AGND (Pin 17): Analog Ground. LADCOM (Pin 18): Attenuator Ladder Common. Normally tied to GND. IOUT B (Pin 19): Complementary DAC Output Current. Fullscale output current occurs when all data bits are 0s. IOUT A (Pin 20): DAC Output Current. Full-scale output current occurs when all data bits are 1s. COMP1 (Pin 21): Current Source Control Amplifier Compensation. Bypass to VSS with 0.1µF. COMP2 (Pin 22): Internal Bypass Point. Bypass to VSS with 0.1µF. VSS (Pin 23): Negative Supply Voltage. Nominal value is – 5V. DGND (Pin 24): Digital Ground. VDD (Pin 25): Positive Supply Voltage. Nominal value is 5V. CLK (Pin 26): Clock Input. Data is latched and the output is updated on positive edge of clock. DB15 to DB0 (Pins 27, 28, 1 to 14): Digital Input Data Bits. 4 UW 2-Tone SFDR –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 12.55 1668 G01 fCLOCK = 25Msps fOUT = 1.007MHz AMPLITUDE = 0dBFS = –8.5dBm SFDR = 86dBc fCLOCK = 50Msps fOUT1 = 4.028MHz fOUT2 = 4.419MHz AMPLITUDE 1, 2 = –6dBFS = –14.5dBm SFDR > 77dBc –110 3.2 4.2 FREQUENCY (0.2MHz/DIV) 5.2 1668 G02 Differential Nonlinearity 2.0 1.5 1.0 0.5 0 –0.5 –1.0 –1.5 –2.0 0 32768 16384 49152 DIGITAL INPUT CODE 65535 1668 G04 U U U LTC1668 BLOCK DIAGRA W 5V 0.1µF 25 LTC1668 LADCOM IOUT A 15 REFOUT 2.5V REFERENCE ATTENUATOR LADDER IOUT B 18 20 19 52.3Ω 52.3Ω VREF 0.1µF RSET 2k + – VOUT 1VP-P DIFFERENTIAL 16 IREFIN LSB SWITCHES IFS/8 SEGMENTED SWITCHES FOR DB15–DB12 + – 21 0.1µF 22 0.1µF COMP1 COMP2 VSS 23 –5V 0.1µF AGND 17 IINT CURRENT SOURCE ARRAY ••• ••• INPUT LATCHES DGND 24 CLK 26 CLOCK INPUT DB15 27 16-BIT DATA INPUT ••• DB0 14 1668 BD TI I G DIAGRA W DB0 TO DB15 N–1 tDS CLK tCLKL tPD tCLKH tST N tDH N+1 IOUT A/IOUT B N–1 N 0.1% 1668 TD UW 5 LTC1668 APPLICATIO S I FOR ATIO Theory of Operation The LTC1668 is a high speed current steering 16-Bit DAC made on an advanced BiCMOS process. Precision thin film resistors and well matched bipolar transistors result in excellent DC linearity and stability. A low glitch current switching design gives excellent AC performance at sample rates up to 50Msps. The device is complete with a 2.5V internal bandgap reference and edge triggered latches, and sets a new standard for DAC applications requiring very high dynamic range at output frequencies up to several megahertz. Referring to the Block Diagram, the DAC contains an array of current sources that are steered to IOUTA or IOUTB with NMOS differential current switches. The four most significant bits, DB15 to DB12 are made up of 15 current segments of equal weight. The lower bits, DB11 to DB0 are binary weighted, using a combination of current scaling and a differential resistive attenuator ladder. All bits and segments are precisely matched, both in current weight for DC linearity, and in switch timing for low glitch impulse and low spurious tone AC performance. Setting the Full-Scale Current, IOUTFS The full-scale DAC output current, IOUTFS, is nominally 10mA, and can be adjusted down to 1mA. Placing a resistor, RSET, between the REFOUT pin, and the IREFIN pin sets IOUTFS as follows. The internal reference control loop amplifier maintains a virtual ground at IREFIN by servoing the internal current source, IINT, to sink the exact current flowing into IREFIN. IINT is a scaled replica of the DAC current sources and IOUTFS = 8 • (IINT), therefore: IOUTFS = 8 • (IREFIN) = 8 • (VREF/RSET) (1) For example, if RSET = 2k and is tied to VREF = REFOUT = 2.5V, IREFIN = 2.5/2k = 1.25mA and IOUTFS = 8 • (1.25mA) = 10mA. The reference control loop requires a capacitor on the COMP1 pin for compensation. For optimal AC performance, CCOMP1 should be connected to VSS and be placed very close to the package (less than 0.1"). 6 U For fixed reference voltage applications, CCOMP1 should be 0.1µF or more. The reference control loop small-signal bandwidth is approximately 1/(2π) • CCOMP1 • 80 or 20kHz for CCOMP1 = 0.1µF. Internal Reference Output—REFOUT The onboard 2.5V bandgap voltage reference drives the REFOUT pin. It is trimmed and specified to drive a 2k resistor tied from REFOUT to IREFIN, corresponding to a 1.25mA load (IOUTFS = 10mA). REFOUT has nominal output impedance of 6Ω, or 0.24% per mA, so it must be buffered to drive any additional external load. A 0.1µF capacitor is required on the REFOUT pin for compensation. Note that this capacitor is required for stability, even if the internal reference is not being used. DAC Transfer Function The LTC1668 uses straight binary digital coding. The complementary current outputs, IOUT A and IOUT B, sink current from 0 to IOUTFS. For IOUTFS = 10mA (nominal), IOUT A swings from 0mA when all bits are low (i.e., Code = 0) to 10mA when all bits are high (i.e., Code = 65535) (decimal representation). IOUT B is complementary to IOUT A. IOUT A and IOUT B are given by the following formulas: IOUT A = IOUTFS • (DAC Code/65536) IOUT B = IOUTFS • (65535-DAC Code)/65536 (2) (3) In typical applications, the LTC1668 differential output currents either drive a resistive load directly or drive an equivalent resistive load through a transformer, or as the feedback resistor of an I-to-V converter. The voltage outputs generated by the IOUT A and IOUT B output currents are then: VOUT A = IOUT A • RLOAD VOUT B = IOUT B • RLOAD The differential voltage is: VDIFF = VOUT A – VOUT B = (IOUT A – IOUT B) • (RLOAD) (6) (4) (5) W UU LTC1668 APPLICATIO S I FOR ATIO Substituting the values found earlier for IOUT A, IOUT B and IOUTFS: VDIFF = {2 • DAC Code – 65535)/65536} • 8 • (RLOAD/RSET) • (VREF) (7) From these equations some of the advantages of differential mode operation can be seen. First, any common mode noise or error on IOUT A and IOUT B is cancelled. Second, the signal power is twice as large as in the single-ended case. Third, any errors and noise that multiply times IOUT A and IOUT B, such as reference or IOUTFS noise, cancel near midscale, where AC signal waveforms tend to spend the most time. Fourth, this transfer function is bipolar; e.g. the output swings positive and negative around a zero output at mid-scale input, which is more convenient for AC applications. Note that the term (RLOAD/RSET) appears in both the differential and single-ended transfer functions. This means that the Gain Error of the DAC depends on the ratio of RLOAD to RSET, and the Gain Error tempco is affected by the temperature tracking of RLOAD with RSET. Note also that the absolute tempco of RLOAD is very critical for DC nonlinearity. As the DAC output changes from 0mA to 10mA the RLOAD resistor will heat up slightly, and even a very low tempco can produce enough INL bowing to be significant at the 16-bit level. This effect disappears with medium to high frequency AC signals due to the slow thermal time constant of the load resistor. Analog Outputs The LTC1668 has two complementary current outputs, IOUT A and IOUT B (see DAC Transfer Function). The output impedance of IOUT A and IOUT B (RIOUT A and RIOUT B) is typically 1.1kΩ to LADCOM. (See the Equivalent Analog Output Circuit, Figure 1.) The LADCOM pin is the common connection for the internal DAC attenuator ladder. It usually is tied to analog ground, but more generally it should connect to the same potential as the lead resistors on IOUT A and IOUT B. The LADCOM pin carries a constant current to VSS of approximately 0.32 • (IOUTFS), plus any current that flows from IOUT A and IOUT B through the RIOUT A and RIOUT B resistors. U LTC1668 RIOUT B 1.1k RIOUT A 1.1k LADCOM 18 IOUT A IOUT B 20 52.3Ω 19 52.3Ω 5pF 5pF VSS 23 1668 F01 W UU – 5V Figure 1. Equivalent Analog Output Circuit The specified output compliance voltage range is ± 1V. The DC linearity specifications, INL and DNL, are trimmed and guaranteed on IOUT A into the virtual ground of an I-to-V converter, but are typically very good over the full output compliance range. Above 1V the output current will start to increase as the DAC current steering switch impedance decreases, degrading both DC and AC linearity. Below – 1V, the DAC switches will start to approach the transition from saturation to linear region. This will degrade AC performance first, due to nonlinear capacitance and increased glitch impulse. AC distortion performance is optimal at amplitudes less than ± 0.5VP-P on IOUT A and IOUT B due to nonlinear capacitance and other large-signal effects. At first glance, it may seem counter-intuitive to decrease the signal amplitude when trying to optimize SFDR. However, the error sources that affect AC performance generally behave as additive currents, so decreasing the load impedance to reduce signal voltage amplitude will reduce most spurious signals by the same amount. The LTC1668 is specified to operate with full-scale output current, IOUTFS, from the nominal 10mA down to 1mA. This can be useful to reduce power dissipation or to adjust full-scale value. However, that the LTC1668 DC and AC accuracy is specified only at IOUTFS = 10mA, and DC and AC accuracy will fall off significantly at lower IOUTFS values. At IOUTFS = 1mA, INL and DNL typically degrade to the 14bit to 13-bit level, compared to 16-bit to 15-bit typical accuracy at 10mA IOUTFS. Increasing IOUTFS from 1mA, the 7 LTC1668 APPLICATIO S I FOR ATIO accuracy improves rapidly, roughly in proportion to 1/IOUTFS. The AC performance tends to be less affected by reducing IOUTFS, except for the unavoidable affects on SFDR and THD due to increased INL and DNL. Output Configurations Based on the specific application requirements, the LTC1668 allows a choice of the best of several output configurations. Voltage outputs can be generated by external load resistors, transformer coupling or with an op amp I-to-V converter. Single-ended DAC output configurations use only one of the outputs, preferably IOUT A, to produce a single-ended voltage output. Differential mode configurations use the difference between IOUT A and IOUT B to generate an output voltage, VDIFF, as shown in equation 7. Differential mode gives much better accuracy in most AC applications. Because the DAC chip is the point of interface between the digital input signals and the analog output, some small amount of noise coupling to IOUT A and IOUT B is unavoidable. Most of that digital noise is common mode and is canceled by the differential mode circuit. Other significant digital noise components can be modeled as VREF or IOUTFS noise. In single-ended mode, 5V 0.1µF VDD LTC1668 MINI-CIRCUITS T1–1T IOUT A 16-BIT HIGH SPEED DAC 110Ω IOUT B 50Ω LADCOM AGND DGND CLK DB15 DB0 16 DIGITAL DATA OUT 1 OUT 2 CLK HP8110A DUAL IN PULSE GENERATOR LOW JITTER CLOCK SOURCE HP1663EA CLK LOGIC ANALYZER WITH IN PATTERN GENERATOR 50Ω TO HP3589A SPECTRUM ANALYZER 50Ω INPUT REFOUT 0.1µF RSET 2k IREFIN 2.5V REFERENCE + – COMP1 C1 0.1µF COMP2 C2 0.1µF VSS 0.1µF – 5V Figure 2. AC Characterization Setup 8 U IOUTFS noise is gone at zero scale and is fully present at full scale. In differential mode, IOUTFS noise is cancelled at midscale input, corresponding to zero analog output. Many AC signals, including broadband and multitone communications signals with high peak to average ratios, stay mostly near midscale. Differential transformer-coupled output configurations usually give the best AC performance. An example is the AC Characterization Setup circuit, Figure 2. The advantages of transformer coupling include excellent rejection of common mode distortion and noise over a broad frequency range and convenient differential-to-singleended conversion with isolation or level shifting. Also, as much as twice the power can be delivered to the load, and impedance matching can be accomplished by selecting the appropriate transformer turns ratio. The center tap on the primary side of the transformer is tied to ground to provide the DC current path for IOUT A and IOUT B. For low distortion, the DC average of the IOUT A and IOUT B currents must be exactly equal to avoid biasing the core. This is especially important for compact RF transformers with small cores. The circuit in Figure 2 uses a Mini-Circuits T1-1T RF transformer with a 1:1 turns ratio. The load 1668 F02 W UU LTC1668 APPLICATIO S I FOR ATIO IOUT A LTC1668 IOUT B LADCOM 200Ω Figure 3. Unipolar Buffered Voltage Output Digital Interface The LTC1668 has 16 parallel inputs that are latched on the rising edge of the clock input. They accept CMOS levels from either 5V or 3.3V logic and can accept clock rates of up to 50MHz. Referring to the Timing Diagram and Block Diagram, the data inputs go to master-slave latches that update on the rising edge of the clock. The input logic thresholds, VIH = 2.4V min, VIL = 0.8V max, work with 3.3V or 5V CMOS levels over temperature. The guaranteed setup time, tDS, is 8ns minimum and the hold time, tDH, is 4ns minimum. The minimum clock high and low times are guaranteed at 6ns and 8ns, respectively. These specifications allow the LTC1668 to be clocked at up to 50Msps minimum. For best AC performance, the data and clock waveforms need to be clean and free of undershoot and overshoot. Clock and data interconnect lines should be twisted pair, coax or microstrip, and proper line termination is important. If the digital input signals to the DAC are considered as analog AC voltage signals, they are rich in spectral components over a broad frequency range, usually including the output signal band of interest. Therefore, any direct coupling of the digital signals to the analog output will produce spurious tones that vary with the exact digital input pattern. Clock jitter should be minimized to avoid degrading the noise floor of the device in AC applications, especially where high output frequencies are being generated. Any noise coupling from the digital inputs to the clock input will A differential resistor loaded output configuration is shown in the Block Diagram. It is simple and economical, but it can drive only differential loads with impedance levels and amplitudes appropriate for the DAC outputs. The recommended single-ended resistor loaded configuration is essentially the same circuit as the differential resistor loaded, case—simply use the IOUT A output, referred to ground. Rather than tying the unused IOUT B output to ground, it is preferred to load it with the equivalent RLOAD of IOUT A. Then IOUT B will still swing with a waveform complementary to IOUT A. Adding an op amp differential to single-ended converter circuit to the differential resistor loaded output gives the circuit of Figure 10. This circuit complements the capabilities of the transformer-coupled application at lower frequencies, since available op amps can deliver good AC distortion performance at signal frequencies of a few MHz down to DC. The optional capacitor adds a single real pole of filtering, and helps reduce distortion by limiting the high frequency signal amplitude at the op amp inputs. The circuit swings ±1V around ground. Figure 3 shows a simplified circuit for a single-ended output using I-to-V converter to produce a unipolar buffered voltage output. This configuration typically has the best DC linearity performance, but its AC distortion at higher frequencies is limited by U1’s slewing capabilities. + – resistance on IOUT A and IOUT B is equivalent to a single differential resistor of 50Ω, and the 1:1 turns ratio means the output impedance from the transformer is 50Ω. Note that the load resistors are optional, and they dissipate half of the output power. However, in lab environments or when driving long transmission lines it is very desirable to have a 50Ω output impedance. This could also be done with a 50Ω resistor at the transformer secondary, but putting the load resistors on IOUT A and IOUT B is preferred since it reduces the current through the transformer. At signal frequencies lower than about 1MHz, the transformer core size required to maintain low distortion gets larger, and at some lower frequencies this becomes impractical. U COUT RFB 200Ω IOUTFS 10mA U1 LT®1812 VOUT 0V TO 2V 1668 F03 W UU 9 LTC1668 APPLICATIO S I FOR ATIO cause phase modulation of the clock signal and the DAC waveform, and can produce spurious tones. It is normally best to place the digital data transitions near the falling clock edge, well away from the active rising clock edge. Because the clock signal contains spectral components only at the sampling frequency and its multiples, it is usually not a source of in band spurious tones. Overall, it is better to treat the clock as you would an analog signal and route it separately from the digital data input signals. The clock trace should be routed either over the analog ground plane or over its own section of the ground plane. The clock line needs to have accurately controlled impedance and should be well terminated near the LTC1668. Printed Circuit Board Layout Considerations— Grounding, Bypassing and Output Signal Routing The close proximity of high frequency digital data lines and high dynamic range, wide-band analog signals makes clean printed circuit board design and layout an absolute necessity. Figures 5 to 9 are the printed circuit board layers for an AC evaluation circuit for the LTC1668. Ground planes should be split between digital and analog sections as shown. All bypass capacitors should have minimum trace length and be ceramic 0.1µF or larger with low ESR. 10 U Bypass capacitors are required on VSS, VDD and REFOUT, and all connected to the AGND plane. The COMP2 pin ties to a node in the output current switching circuitry, and it requires a 0.1µF bypass capacitor. It should be bypassed to VSS along with COMP1. The AGND and DGND pins should both tie directly to the AGND plane, and the tie point between the AGND and DGND planes should nominally be near the DGND pin. LADCOM should either be tied directly to the AGND plane or be bypassed to AGND. The IOUT A and IOUT B traces should be close together, short, and well matched for good AC CMRR. The transformer output ground should be capable of optionally being isolated or being tied to the AGND plane, depending on which gives better performance in the system. Suggested Evaluation Circuit Figure 4 is the schematic and Figures 5 to 9 are the circuit board layouts for a suggested evaluation circuit, DC245A. The circuit can be programmed with component selection and jumpers for a variety of differentially coupled transformer output and differential and single-ended resistor loaded output configurations. W UU J1 EXTREF 5V JP2 R1 10Ω TP1 TP10 TESTPOINT BLK C1 0.1µF JP1 1 2 4 R4 6 C3 0.1µF C18 0.1µF 3 5 R2 200Ω TP3 TESTPOINT WHT TP2 TESTPOINT WHT 2.5VREF R3 1.91k 0.1% 5V LT1460DCS8-2.5 2 VIN VOUT 6 4 R5 +5VD LTC1668-28 16 16 27 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 26 CLK J6 EXTCLK GROUND PLANE TIE POINT DB0 DB1 DB2 DGND DB3 24 DB4 AGND C12 22pF DB5 VDD 17 R9 50Ω 0.1% 5V C10 0.1µF –5V C11 0.1µF DB6 VSS 25 DB7 23 JP6 JP7 DB8 C7 0.1µF C8 0.1µF COMP2 DB9 22 C8 0.1µF COMP1 DB10 LADCOM 21 DB11 18 JP5 1 J5 IOUT B JP8 C9 0.1µF 6 MINICIRCUITS T1–1T DB12 TP5 TESTPOINT WHT DB13 R7 110Ω IOUT B DB14 19 IOUT A DB15 (MSB) 20 TP4 C4 TESTPOINT WHT 15 14 13 12 11 10 9 22Ω RN6 1 16 15 14 13 12 11 10 9 22Ω 1 2 R12 49.9Ω 3 1% 2 3 4 5 6 7 8 REFIN REFOUT JP3 JP4 3 T1 4 J4 VOUT 2 R8 C5 15 RN5 1 2 3 4 5 6 7 8 C17 0.1µF J2 IOUT A R6 AMP 102159-9 +5VD 4 3 6 5 8 7 APPLICATIO S I FOR ATIO 10 9 14 13 16 15 18 17 20 19 22 21 24 23 OPTIONAL SIP PULL-UP/ PULL-DOWN RESISTORS (NOT INSTALLED) OPTIONAL SIP PULL-UP/ PULL-DOWN RESISTORS (NOT INSTALLED) R10 50Ω 0.1% 26 25 C12 22pF 28 27 30 29 1668 F04 32 31 34 33 36 35 38 37 40 39 AGND DGND JP9 +5VD J8 J9 J7 TP6 TESTPOINT RED +5VA –5V TP7 TESTPOINT RED TP8 TESTPOINT RED + J11 C21 0.1µF C23 0.1µF C15 10µF 25V + TP9 TESTPOINT BLK Figure 4. Suggested Evaluation Circuit + J10 C19 0.1µF C14 10µF 25V C22 0.1µF C20 0.1µF C16 10µF 25V U LTC1668 12 11 W 2 1 UU C2 0.1µF GND 11 LTC1668 APPLICATIO S I FOR ATIO U W UU Figure 5. Suggested Evaluation Circuit Board—Silkscreen Figure 6. Suggested Evaluation Circuit Board—Component Side 12 LTC1668 APPLICATIO S I FOR ATIO U W UU Figure 7. Suggested Evaluation Circuit Board—GND Plane Figure 8. Suggested Evaluation Circuit Board—Power Plane 13 LTC1668 APPLICATIO S I FOR ATIO Figure 9. Suggested Evaluation Circuit Board—Solder Side 14 U W UU LTC1668 PACKAGE DESCRIPTIO U Dimensions in millimeters (inches) unless otherwise noted. G Package 28-Lead Plastic SSOP (0.209) (LTC DWG # 05-08-1640) 10.07 – 10.33* (0.397 – 0.407) 28 27 26 25 24 23 22 21 20 19 18 17 16 15 7.65 – 7.90 (0.301 – 0.311) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 5.20 – 5.38** (0.205 – 0.212) 1.73 – 1.99 (0.068 – 0.078) 0° – 8° 0.13 – 0.22 (0.005 – 0.009) 0.55 – 0.95 (0.022 – 0.037) 0.65 (0.0256) BSC NOTE: DIMENSIONS ARE IN MILLIMETERS *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.152mm (0.006") PER SIDE **DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.254mm (0.010") PER SIDE 0.25 – 0.38 (0.010 – 0.015) 0.05 – 0.21 (0.002 – 0.008) G28 SSOP 1098 15 LTC1668 TYPICAL APPLICATIO U 5V 0.1µF VDD LTC1668 500Ω 200Ω IOUT A 16-BIT HIGH SPEED DAC REFOUT 0.1µF RSET 2k IREFIN 2.5V REFERENCE + – COMP1 0.1µF 0.1µF COMP2 VSS AGND DGND CLK – LT®1812 IOUT B COPT LADCOM 200Ω 500Ω 52.3Ω + VOUT ± 1V 10dBm DB15 DB0 1668 F10 52.3Ω 0.1µF – 5V CLOCK 16-BIT DATA INPUT INPUT Figure 10. High Speed Buffered VOUT DAC RELATED PARTS PART NUMBER LTC1406 LTC1414 LTC1420 LTC1604 DESCRIPTION 8-Bit, 20Msps ADC 14-Bit, 2.2Msps ADC 12-Bit, 10Msps ADC 16-Bit, 333ksps ADC COMMENTS Undersampling Capability Up to 70MHz Input 84dB SFDR at 1.1MHz fIN 72dB SINAD at 5MHz fIN 16-Bit, No Missing Codes, 90dB SINAD, –100dB THD 16 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com 1668i LT/TP 0200 4K • PRINTED IN USA © LINEAR TECHNOLOGY CORPORATION 2000