công cụ tìm kiếm bảng dữ liệu linh kiện điện tử |
|
ADS821 bảng dữ liệu(PDF) 11 Page - Texas Instruments |
|
|
ADS821 bảng dữ liệu(HTML) 11 Page - Texas Instruments |
11 / 20 page www.ti.com ADS821 11 SBAS040B APPLICATIONS DRIVING THE ADS821 The ADS821 has a differential input with a common-mode of +2.25V. For AC-coupled applications, the simplest way to create this differential input is to drive the primary winding of a transformer with a single-ended input. A differential output is created on the secondary if the center tap is tied to the common-mode (CM) voltage of +2.25V, as per Figure 5. This transformer-coupled input arrangement provides good high- frequency AC performance. It is important to select a trans- former that gives low distortion and does not exhibit core saturation at full-scale voltage levels. Since the transformer does not appreciably load the ladder, there is no need to buffer the CM output in this instance. In general, it is advisable to keep the current draw from the CM output pin below 0.5 µA to avoid nonlinearity in the internal reference ladder. A FET input operational amplifier such as the OPA130 can provide a buffered reference for driving external circuitry. The analog IN and IN inputs should be bypassed with 22pF capacitors to minimize track-and-hold glitches and to im- prove high-input frequency performance. Figure 6 shows an AC-coupled single-ended input interface circuit using the low-cost, current feedback OPA694 as the active gain stage. When testing this configuration in gains of +4, +5.8, and +8.2, it was noted that reducing the feedback resistor of the OPA694 from the typical 402 Ω to 360Ω resulted in a wider bandwidth, thus improving distortion at higher gains. The gain resistor was scaled to 120 Ω, 75Ω, and 50 Ω for each of the three gain settings. The two 330Ω resistors set the RC time constant and the values can be varied, although higher values will have the effect of moving the corner frequency of the created high-pass filter down. In Figure 6, the –3dB point is set at 4.2kHz. Figure 7 illustrates another possible low-cost interface circuit that utilizes resistors and capacitors in place of a transformer. Depending on the signal bandwidth, the component values should be carefully selected in order to maintain the perfor- mance outlined in the data sheet. The input capacitors, CIN, and the input resistors, RIN, create a high-pass filter with the lower corner frequency at fC = 1/(2πRINCIN). The corner frequency can be reduced by either increasing the value of RIN or CIN. If the circuit operates with a 50Ω or 75Ω imped- ance level, the resistors are fixed and only the value of the capacitor can be increased. Usually AC-coupling capacitors are electrolytic or tantalum capacitors with values of 1mF or higher. It should be noted that these large capacitors become inductive with increased input frequency, which could lead to signal amplitude errors or oscillation. To maintain a low AC- coupling impedance throughout the signal band, a small value (e.g. 1 µF) ceramic capacitor could be added in parallel with the polarized capacitor. Capacitors CSH1 and CSH2 are used to minimize current glitches resulting from the switching in the input track-and- hold stage and to improve signal-to-noise performance. These capacitors can also be used to establish a low-pass filter and effectively reduce the noise bandwidth. In order to create a real pole, resistors RSER1 and RSER2 were added in series with each input. The cut off frequency of the filter is determined by fC = 1/(2πRSER • (CSH + CADC)) where RSER is the resistor in series with the input, CSH is the external capacitor from the input to ground, and CADC is the internal input capacitance of the A/D converter (typically 4pF). Resistors R1 and R2 are used to derive the necessary com- mon-mode voltage from the buffered top and bottom refer- ences. The total load of the resistor string should be selected FIGURE 5. AC-Coupled, Single-Ended to Differential Drive Circuit Using a Transformer. FIGURE 6. Low-Cost, AC-Coupled, Single-Ended Input Circuit. Mini-Circuits T T1-6-KK81 or Equivalent 22 26 27 CM IN IN ADS821 AC Input Signal 22pF 22pF 0.1 µF ADS821 A 1 V IN R G OPA694 0.1 || 2.2 0.1 || 2.2 +5V –5V I/O 360 Ω 330 Ω 330 Ω IN 27 26 22 CM +2.25V IN 22pF 49.9 Ω 0.1 µF 0.1 µF 0.1 µF |
Số phần tương tự - ADS821 |
|
Mô tả tương tự - ADS821 |
|
|
Link URL |
Chính sách bảo mật |
ALLDATASHEET.VN |
Cho đến nay ALLDATASHEET có giúp ích cho doanh nghiệp của bạn hay không? [ DONATE ] |
Alldatasheet là | Quảng cáo | Liên lạc với chúng tôi | Chính sách bảo mật | Trao đổi link | Tìm kiếm theo nhà sản xuất All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |