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ADS5421R Datasheet(PDF) 10 Page - Burr-Brown (TI) |
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ADS5421R Datasheet(HTML) 10 Page - Burr-Brown (TI) |
10 / 21 page ADS5421 10 SBAS237D www.ti.com FIGURE 3. Using the THS4502 Differential Amplifier (Gain = 1) to Drive the ADS5421 in a DC-Coupled Configuration. For applications that use op amps to drive the ADC, it is recommended that a series resistor be added between the amplifier output and the converter inputs. This will isolate the capacitive input of the converter from the driving source and avoid gain peaking, or instability; furthermore, it will create a 1st-order, low-pass filter in conjunction with the specified input capacitance of the ADS5421. Its cutoff frequency can be adjusted further by adding an external shunt capacitor from each signal input to ground. The optimum values of this RC network, however, depend on a variety of factors, includ- ing the ADS5421 sampling rate, the selected op amp, the interface configuration, and the particular application (time domain versus frequency domain). Generally, increasing the size of the series resistor and/or capacitor will improve the SNR, however, depending on the signal source, large resis- tor values may be detrimental to the harmonic distortion performance. In any case, the use of the RC network is optional but optimizing the values to adapt to the specific application is encouraged. ANALOG INPUT DRIVER CONFIGURATIONS The following section provides some principal circuit sugges- tions on how to interface the analog input signal to the ADS5421. Applications that have a requirement for DC- coupling a new differential amplifier, such as the THS4502, can be used to drive the ADS5421, as shown in Figure 3. The THS4502 amplifier allows a single-ended to differential con- version to be performed easily, which reduces component cost. In addition, the VCM pin on the THS4502 can be directly tied to the common-mode pin (CM) of the ADS5421 in order to set up the necessary bias voltage for the converter inputs. As shown in Figure 3, the THS4502 is configured for unity gain. If required, higher gain can easily be configured, and a low-pass filter can be created by adding small capacitors (e.g., 10pF) in parallel to the feedback resistors. Due to the THS4502 driving a capacitive load, small series resistors in the output ensure stable operation. Further details of this and other functions of the THS4502 may be found in its product datasheet located at the Texas Instruments web site (www.ti.com). In general, differential amplifiers provide for a high-performance driver solution for baseband applications, and different differential amplifier models can be selected depending on the system requirements. TRANSFORMER-COUPLED INTERFACE CIRCUITS If the application allows for AC-coupling but requires a signal conversion from a single-ended source to drive the ADS5421 differentially, using a transformer offers a number of advan- tages. As a passive component, it does not add to the total noise, and by using a step-up transformer, further signal amplification can be realized. As a result, the signal swing of the amplifier driving the transformer can be reduced, leading to an increased headroom for the amplifier and improved distortion performance. A transformer interface solution is given in Figure 4. The input signal is assumed to be an IF and bandpass filtered prior to the IF amplifier. Dedicated IF amplifiers are com- monly fixed-gain blocks and feature a very high bandwidth, low-noise figure, and a high intercept point, but at the expense of high quiescent currents, which are often around 100mA. The IF amplifier may be AC-coupled, or directly connected to the primary side of the transformer. A variety of miniature RF transformers are readily available from different manufacturers, (e.g., Mini-Circuits, Coilcraft, or Trak). For selection, it is important to carefully examine the application requirements and determine the correct model, the desired impedance ratio, and frequency characteristics. Furthermore, the appropriate model must support the targeted distortion level and should not exhibit any core saturation at full-scale voltage levels. The transformer center tap can be directly tied to the CM pin of the converter because it does not appreciably load the ADC reference (see Figure 4). The value of termina- tion resistor RT must be chosen to satisfy the termination requirements of the source impedance (RS). It can be calcu- lated using the equation RT = n2 • RS to ensure proper impedance matching. 0.1 µF 10pF(1) 10pF(1) ADS5421 THS4502 25 Ω R S 392 Ω 392 Ω 392 Ω 25 Ω 22pF 412 Ω V CM IN IN CM +5V +5V –5V NOTES: Supply bypassing not shown. (1) Optional. 56.2 Ω |
Similar Part No. - ADS5421R |
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Similar Description - ADS5421R |
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