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SPT7935SIT Datasheet(PDF) 6 Page - Fairchild Semiconductor |
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SPT7935SIT Datasheet(HTML) 6 Page - Fairchild Semiconductor |
6 / 8 page 6 7/12/00 SPT7935 COMMON MODE VOLTAGE REFERENCE CIRCUIT The SPT7935 has an on-board common mode voltage refer- ence 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. 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. Table I – Sample Rate Settings Sample Rate (MHz) Bias 1 ( µA) Bias 2 ( µA) 1 20 3.5 5 50 6.5 10 80 8.0 20 90 9.5 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. Figure 3 illustrates a solution (based on operational amplifi- ers) 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 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. REFERENCES 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. VCM R3 (R3)/2 + – Input Voltage ( ±0.5 V) R3 R R2 R2 + – – + R RR 51 Ω 51 Ω R ADC VIN+ VIN– 15 pF 51 Ω Figure 3 – DC-Coupled Single Ended to Differential Conversion (Power Supplies and Bypassing are Not Shown) |
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