SPT7935 12-BIT, 20 MSPS, 79 mW A/D CONVERTER FEATURES APPLICATIONS • • • • • • • • • • • • • • 12-Bit, 20 MSPS Analog-to-Digital Converter Monolithic CMOS Internal Track-and-Hold Low Input Capacitance: 1.4 pF Low Power Dissipation: 79 mW 2.8 – 3.6 V Power Supply Range TTL-Compatible Outputs GENERAL DESCRIPTION The SPT7935 12-bit, 20 MSPS analog-to-digital converter has a pipelined converter architecture built in a CMOS process. It delivers high performance with a typical power dissipation of only 79 mW. With low distortion and high dynamic range, this device offers the performance needed CCD Imaging Cameras and Sensors Medical Imaging RF Communications Document and Film Scanners Electro-Optics Transient Signal Analysis Handheld Equipment for imaging, multimedia, telecommunications and instrumentation applications. The SPT7935 is available in a 44-lead Thin Quad Flat Pack (TQFP) package in the industrial temperature range (–40 to +85 °C). BLOCK DIAGRAM ADC + DAC – G=2 D<1…0> Pipeline Stage VIN+ VIN– VREF+ Stage 1 Stage 2 Stage 9 Stage 10 VREF– CLK Clock Driver Digital Delays, Error Correction and Output 12 Digital Output (D0 – D11) 2-Bit ADC ABSOLUTE MAXIMUM RATINGS (Beyond which damage may occur)1 25 °C Supply Voltages VDD1 .................................................................... –0.5 V to +6 V VDD2 .................................................................... –0.5 V to +6 V VDD3 .................................................................... –0.5 V to +6 V Temperature Operating Temperature ............................. –40 to +85 °C Storage Temperature ............................... –65 to +125 °C Input Voltages Analog Input ................................. –0.5 V to (VDD +0.5 V) Digital Input .................................. –0.5 V to (VDD +0.5 V) VREF+ .......................................... –0.5 V to (VDD +0.5 V) VREF– .......................................... –0.5 V to (VDD +0.5 V) CLK .............................................. –0.5 V to (VDD +0.5 V) Note: 1. Operation at any Absolute Maximum Rating is not implied. See Electrical Specifications for proper nominal applied conditions in typical applications. ELECTRICAL SPECIFICATIONS TA=TMIN–TMAX , VDD1=VDD2=VDD3=3.3 V, VREF–=1.0 V, VREF+=2.0 V, Common Mode Voltage=1.65 V, ƒCLK=20 MSPS, Bias 1=90 µA, Bias 2=9.5 µA, Differential Input, Duty Cycle=50%, unless otherwise specified. PARAMETERS TEST CONDITIONS TEST LEVEL DC Accuracy Resolution Differential Linearity Integral Linearity No Missing Codes Analog Input Input Voltage Range (Differential) Common Mode Input Voltage Input Capacitance Input Bandwidth (Large Signal) Offset (Mid-scale) Gain Error Reference Voltages Reference Input Voltage Range (VREF+ – VREF–) Negative Reference Voltage (VREF–) Positive Reference Voltage (VREF+) Common Mode Output Voltage (VCM) VREF+ Current VREF– Current MIN IO = –1 µA Switching Performance Maximum Conversion Rate Pipeline Delay (See Timing Diagram) Aperture Delay Time (TAP) Aperture Jitter Time UNITS Bits LSB LSB IV IV V V V V ±0.6 1.2 ±1.0 1.65 1.4 120 ±1.0 0.3 ±1.7 1.9 V V pF MHz % FSR % FSR IV 0.6 1.0 1.7 V IV IV VI V V 0.9 1.9 1.3 1.0 2.0 1.65 35 –25 1.3 2.6 1.8 V V V µA µA VI IV 20 V V Dynamic Performance Effective Number of Bits ƒIN = 5.0 MHz ƒIN = 10.0 MHz Signal-To-Noise Ratio ƒIN = 5.0 MHz ƒIN = 10.0 MHz Total Harmonic Distortion ƒIN = 5.0 MHz ƒIN = 10.0 MHz MAX 12 ±0.6 ±3.0 Guaranteed V V VI VIN+=VIN–=VCM SPT7935 TYP 7.5 MHz Clocks 5 10 ns ps-rms VI V 9.2 9.8 9.0 Bits Bits VI V 59 62 58 dB dB VI V –68 –60 –61 dB dB SPT7935 2 7/12/00 ELECTRICAL SPECIFICATIONS TA=TMIN–TMAX , VDD1=VDD2=VDD3=3.3 V, VREF–=1.0 V, VREF+=2.0 V, Common Mode Voltage=1.65 V, ƒCLK=20 MSPS, Bias 1=90 µA, Bias 2=9.5 µA, Differential Input, Duty Cycle=50%, unless otherwise specified. PARAMETERS TEST CONDITIONS TEST LEVEL Dynamic Performance–Continued Signal-To-Noise and Distortion ƒIN = 5 MHz ƒIN = 10 MHz Spurious Free Dynamic Range ƒIN = 5.0 MHz ƒIN = 10.0 MHz Differential Phase Differential Gain Digital Inputs Logic 1 Voltage Logic 0 Voltage Maximum Input Current Low Maximum Input Current High Input Capacitance Digital Outputs Logic 1 Voltage Logic 0 Voltage CLK to Output Delay Time (tD) VIN = GND VIN = VDD IO = -2 mA IO = +2 mA Power Supply Requirements Supply Voltages VDD1, VDD2, VDD3 Supply Current IDD Power Dissipation Power Supply Rejection Ratio (PSRR) TEST LEVEL CODES TEST LEVEL I All electrical characteristics are subject to the following conditions: All parameters having II min/max specifications are guaranteed. The Test Level column indicates the specific III device testing actually performed during proIV duction and Quality Assurance inspection. Any blank section in the data column indiV cates that the specification is not tested at the specified condition. VI MIN SPT7935 TYP MAX UNITS VI V 57 61 56 dB dB VI V V V 62 70 61 0.2 0.5 dB dB Degrees % VI VI VI VI V 80% VDD VI VI IV 85% VDD IV VI VI V 1.8 20% VDD ±1 µA ±1 µA pF 4 95% VDD 0.1 8 0.4 12 V V ns 2.8 3.3 3.6 V 24 79 67 30 100 mA mW dB TEST PROCEDURE 100% production tested at the specified temperature. 100% production tested at TA = +25 °C, and sample tested at the specified temperatures. QA sample tested only at the specified temperatures. Parameter is guaranteed (but not tested) by design and characterization data. Parameter is a typical value for information purposes only. 100% production tested at TA = +25 °C. Parameter is guaranteed over specified temperature range. SPT7935 3 7/12/00 TYPICAL PERFORMANCE CHARACTERISTICS THD, SNR, SINAD vs Sample Rate 80 80 70 70 60 THD, SNR, SINAD (dB) THD, SNR, SINAD (dB) THD, SNR, SINAD vs Input Frequency THD SNR SINAD 50 40 THD THD SNR SNR 60 SINAD SINAD 50 40 30 30 20 20 100 10 0 102 101 Input Frequency (MHz) 102 101 Sample Rate (MSPS) Note: Bias1 and Bias2 currents optimized for each sample rate. THD, SNR, SINAD vs Temperature Power Dissipation vs Sample Rate 70 150 125 Power Dissipation (mW) THD 66 64 SNR 62 SINAD 60 100 75 50 25 58 0 56 0 25 10 0 70 Temperature (°C) 102 101 Sample Rate (MSPS) Note: Bias1 and Bias2 optimized for each sample rate. Bias 1 Voltage vs Bias 1 Current Bias 2 Voltage vs Bias 2 Current 3.4 0.90 3.2 0.85 3.0 0.80 2.8 VBias2 (V) VBias1 (V) THD, SNR, SINAD (dB) 68 IBias1 VBias1 30 2.19 60 2.53 90 2.79 120 3.00 150 3.22 2.6 2.4 IBias2 3 6 9 12 15 0.75 0.70 VBias2 0.6975 0.7535 0.796 0.8295 0.8595 0.65 2.2 2.0 0.60 0 30 60 90 120 150 180 0 IBias1 (µA) 3 6 9 12 15 18 IBias2 (µA) SPT7935 4 7/12/00 Figure 1 – Timing Diagram Sampling Points N N-1 N+1 tAP N+2 N+6 N+8 N+7 AIN CLK tD DOUT N-2 N-1 N GENERAL DESCRIPTION TYPICAL INTERFACE CIRCUIT The SPT7935 is an ultra-low power, 12-bit, 20 MSPS ADC. It has a pipelined architecture and incorporates digital error correction of the 11 most significant bits. This error correction ensures good linearity performance for input frequencies up to Nyquist. The inputs are fully differential, making the device insensitive to system-level noise. This device can also be used in a single-ended mode. (See analog input section.) With the power dissipation roughly proportional to the sampling rate, this device is ideal for very low power applications in the range of 1 to 20 MSPS. The SPT7935 requires few external components to achieve the stated operation and performance. Figure 2 shows the typical interface requirements when using the SPT7935 in normal circuit operation. The following sections provide a description of the functions and outline critical performance criteria to consider for achieving the optimal device performance. ANALOG INPUT The input of the SPT7935 can be configured in various ways depending on if a single-ended or differential, AC- or DCcoupled input is desired. Figure 2 – Typical Interface Circuit +3.3 V CLK In (3 V Logic) 4.7 µF Ref– In 10 µF + .01 µF + +3.3 V (+1.15 V) Ref+ In +3.3 V Digital .01 µF (+2.15 V) + .01 µF 11 1 N/C GND CLK VDD3 VDD2 VDD1 VDD2 VDD1 VDD1 N/C VREF+ 12 VREF– 4.7 µF 0.1 µF N/C VDD3 44 (LSB) D0 D1 N/C 90 µA D2 GND Bias1 9.5 µA (+1.65 V) RF In 68 pF 51 VCM D5 GND D6 VIN+ D7 VIN– D8 GND GND 23 33 AGND Interfacing 3 V Logic D9 D10 Minicircuit T1-6T D4 D11 22 D3 U1 SPT7935 Bias2 .01 µF Decoupling Cap 34 (MSB) FB DGND Notes: All VDD1, VDD2 and VDD3 should be tied together. FB = Ferrite Bead; must be placed as close to U1 as possible. SPT7935 5 7/12/00 The AC coupled input is most conveniently implemented using a transformer with a center tapped secondary winding. The center tap is connected to the VCM pin as shown in figure 2. To obtain low distortion, it is important that the selected transformer does not exhibit core saturation at the full-scale voltage. Proper termination of the input is important for input signal purity. A small capacitor across the inputs attenuates kickback noise from the internal sample and hold. COMMON MODE VOLTAGE REFERENCE CIRCUIT The SPT7935 has an on-board common mode voltage reference circuit (VCM). It is typically one-half of the supply voltage and can drive loads of up to 20 µA. This circuit is commonly used to drive the center tap of the RF transformer in fully differential applications. For single-ended applications, this output can be used to provide the level shifting required for the single-to-differential converter conversion circuit. Figure 3 illustrates a solution (based on operational amplifiers) that can be used if a DC coupled single-ended input is desired. The selection criteria of the buffer op-amps is as follows: – Open loop gain >75 dB – Gain bandwidth product >50 MHz – Total harmonic distortion ≤–75 dB – Signal to noise ratio >75 dB BIAS CURRENT CIRCUITS The bias currents suggested (Bias 1 and Bias 2 in figure 2) optimize device performance for the stated sample rate of 20 MSPS. To achieve the best dynamic performance when operating the device at sample rates other than 20 MSPS, the bias current levels should be adjusted. Table I shows the settings for Bias 1 and Bias 2 for selected sample rates. The “Bias Voltage vs Bias Current” graphs on page 4 show the relationship between the bias current and the bias voltage. POWER SUPPLIES AND GROUNDING The SPT7935 is operated from a single power supply in the range of 2.8 to 3.6 volts. Nominal operation is suggested to be 3.3 volts. All power supply pins should be bypassed as close to the package as possible. The analog and digital grounds should be connected together with a ferrite bead as shown in the typical interface circuit and as close to the ADC as possible. Table I – Sample Rate Settings Sample Rate (MHz) 1 5 10 20 REFERENCES Bias 1 (µA) 20 50 80 90 Bias 2 (µA) 3.5 6.5 8.0 9.5 The SPT7935 has a differential analog input. The voltages applied to the VREF+ and VREF- pins determine the input voltage range and are equal to ±(VREF+ – VREF–). This voltage range will be symmetric about the common mode voltage. Externally generated reference voltages must be connected to these pins. (See figure 2, Typical Interface Circuit.) For best performance, these voltages should be symmetrical about the midpoint of the supply voltage. Figure 3 – DC-Coupled Single Ended to Differential Conversion (Power Supplies and Bypassing are Not Shown) R3 R3 R – VCM ADC + Input Voltage (±0.5 V) (R3)/2 R 51 Ω – R2 + VIN+ 15 pF R2 VIN– 51 Ω R 51 Ω + – R R SPT7935 6 7/12/00 CLOCK DIGITAL OUTPUTS The SPT7935 accepts a low voltage CMOS logic level at the CLK input. The duty cycle of the clock should be kept as close to 50% as possible. Because consecutive stages in the ADC are clocked in opposite phase to each other, a non-50% duty cycle reduces the settling time available for every other stage and thus potentially causing a degradation of dynamic performance. The digital output data appears in an offset binary code at 3.3 V CMOS logic levels. A negative full scale input results in an all zeros output code (000…0). A positive full scale input results in an all 1’s code (111…1). The output data is available 7.5 clock cycles after the data is sampled. The input signal is sampled on the high to low transition of the input clock. Output data should be latched on the low to high clock transition as shown in figure 1, the Timing Diagram. The output data is invalid for the first 20 clock cycles after the device is powered up. For optimal performance at high input frequencies, the clock should have low jitter and fast edges. The rise/fall times should be kept shorter than 2 ns. Overshoot and undershoot should be avoided. Clock jitter causes the noise floor to rise proportional to the input frequency. Because jitter can be caused by crosstalk on the PC board, it is recommended that the clock trace be kept as short as possible and standard transmission line practices be followed. EVALUATION BOARD The EB7935 Evaluation Board is available to aid designers in demonstrating the full performance capability of the SPT7935. The board includes an on-board clock driver, adjustable voltage references, adjustable bias current circuits, single-to-differential input buffers with adjustable levels, a single-to-differential transformer (1:1), digital output buffers and 3.3/5 V adjustable logic outputs. An application note (AN7935) is also available which describes the operation of the evaluation board and provides an example of the recommended power and ground layout and signal routing. Contact the factory for price and availability. PACKAGE OUTLINE 44L TQFP A B INCHES C D MILLIMETERS SYMBOL MIN MAX MIN MAX A 0.472 Typ 12.00 Typ B 0.394 Typ 10.00 Typ C 0.394 Typ 10.00 Typ D 0.472 Typ 12.00 Typ E 0.031 Typ F 0.012 0.018 0.300 0.45 G 0.053 0.057 1.35 1.45 H 0.002 0.006 0.05 0.15 0.030 0.450 0.750 0.80 Typ I 0.018 J 0.039 Typ 1.00 Typ K 0-7° 0-7° Index Pin 1 E F G K I H J SPT7935 7 7/12/00 PIN ASSIGNMENTS PIN FUNCTIONS 34 37 D9 38 35 D6 39 D8 D5 40 D7 D4 41 36 D2 43 42 D3 D1 44 D0 (LSB) VDD3 Name Function VIN+, VIN– Analog Inputs VREF+, VREF– External Reference Inputs CLK Input Clock VCM Common Mode Output Voltage (1.65 V typ) Bias Current (90 µA typ) VDD2 6 28 GND Bias 2 Bias Current (9.5 µA typ) VDD1 7 27 GND D0 – D11 Digital Outputs (D0 = LSB) VDD1 8 26 GND GND Analog Ground VDD1 9 25 GND VREF- 10 24 GND VREF+ 11 23 GND VDD1 VDD2 VDD3 Analog Power Supply Digital Power Supply Digital Output Power Supply N/C No Connect Pins. Recommended to connect to analog ground. N/C GND Bias 1 22 GND VIN- 29 21 5 VIN+ VDD2 GND GND 20 30 19 4 VCM VDD3 Bias 2 GND 18 31 Bias 1 3 17 N/C 16 D11 (MSB) N/C 32 15 2 N/C CLK N/C D10 13 33 14 1 12 GND ORDERING INFORMATION PART NUMBER SPT7935SIT TEMPERATURE RANGE PACKAGE TYPE –40 to +85 °C 44L TQFP SPT7935 8 7/12/00