データシートサーチシステム |
|
ADA4940-1 データシート(PDF) 26 Page - Analog Devices |
|
ADA4940-1 データシート(HTML) 26 Page - Analog Devices |
26 / 30 page ADA4940-1/ADA4940-2 Data Sheet Rev. D | Page 26 of 30 INPUT AND OUTPUT CAPACITIVE AC COUPLING Although the ADA4940-1/ADA4940-2 is best suited to dc- coupled applications, it is nonetheless possible to use it in ac- coupled circuits. Input ac coupling capacitors can be inserted between the source and RG. This ac coupling blocks the flow of the dc common-mode feedback current and causes the ADA4940-1/ADA4940-2 dc input common-mode voltage to equal the dc output common-mode voltage. These ac coupling capacitors must be placed in both loops to keep the feedback factors matched. Output ac coupling capacitors can be placed in series between each output and its respective load. SETTING THE OUTPUT COMMON-MODE VOLTAGE The VOCM pin of the ADA4940-1/ADA4940-2 is internally biased at a voltage approximately equal to the midsupply point, [(+VS) + (−VS)]/2. Relying on this internal bias results in an output common-mode voltage that is within approximately 100 mV of the expected value. In cases where more accurate control of the output common-mode level is required, it is recommended that an external source, or resistor divider (10 kΩ or greater resistors), be used. The output common-mode offset listed in the Specifications section assumes that the VOCM input is driven by a low impedance voltage source. It is also possible to connect the VOCM input to a common-mode level (CML) output of an ADC. However, care must be taken to ensure that the output has sufficient drive capability. The input impedance of the VOCM pin is approximately 250 kΩ. DISABLE PIN The ADA4940-1/ADA4940-2 feature a DISABLE pin that can be used to minimize the quiescent current consumed when the device is not being used. DISABLE is asserted by applying a low logic level to the DISABLE pin. The threshold between high and low logic levels is nominally 1.4 V above the negative supply rail. See Table 5 and Table 8 for the threshold limits. The DISABLE pin features an internal pull-up network that enables the amplifier for normal operation. The ADA4940-1/ ADA4940-2 DISABLE pin can be left floating (that is, no external connection is required) and does not require an external pull-up resistor to ensure normal on operation (see Figure 70). When the ADA4940-1/ADA4940-2 is disabled, the output is high impedance. Note that the outputs are tied to the inputs through the feedback resistors and to the source using the gain resistors. In addition, there are back-to-back diodes on the input pins that limit the differential voltage to 1.2 V. DISABLE AMPLIFIER BIAS CURRENT –VS +VS Figure 70. DISABLE Pin Circuit DRIVING A CAPACITIVE LOAD A purely capacitive load reacts with the bond wire and pin inductance of the ADA4940-1/ADA4940-2, resulting in high frequency ringing in the transient response and loss of phase margin. One way to minimize this effect is to place a resistor in series with each output to buffer the load capacitance. The resistor and load capacitance form a first-order, low-pass filter; therefore, the resistor value must be as small as possible. In some cases, the ADCs require small series resistors to be added on their inputs. Figure 71 illustrates the capacitive load vs. the series resistance required to maintain a minimum 45° of phase margin. 120 0 5 100 10 1000 20 40 60 80 100 +IN –OUT +OUT –FB +FB –IN VOCM 0.1µF RS RS R1 CL CL R2 R4 +2.5V –2.5V R3 VIN LOAD CAPACITANCE (pF) Figure 71. Capacitive Load vs. Series Resistance (LFC SP) |
同様の部品番号 - ADA4940-1_16 |
|
同様の説明 - ADA4940-1_16 |
|
|
リンク URL |
プライバシーポリシー |
ALLDATASHEET.JP |
ALLDATASHEETはお客様のビジネスに役立ちますか? [ DONATE ] |
Alldatasheetは | 広告 | お問い合わせ | プライバシーポリシー | リンク交換 | メーカーリスト 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 |