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LMH6555 データシート(PDF) 11 Page - Texas Instruments |
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LMH6555 データシート(HTML) 11 Page - Texas Instruments |
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11 / 37 page  VIN+ ADC081000/ ADC081500 VCMO SPI RG1 RG2 RF1 RF2 LMH6555 VCM_REF VOUT+ VOUT = 0.8 VPP VIN- VIN a + - RS1 50: RS2 50: RT2 50: RT1 50: 340 mVPP - + OPT LMV321 3.3V OPT LMH6555 www.ti.com SNOSAJ1C – NOVEMBER 2006 – REVISED MARCH 2011 APPLICATION INFORMATION See DEFINITION OF TERMS AND SPECIFICATIONS (ALPHABETICAL ORDER) for definition of terms used. GENERAL The LMH6555 consists of three individual amplifiers: 1. VOUT + driver 2. VOUT − driver 3. The common mode amplifier Being a differential amplifier, the LMH6555 will not respond to the common mode input (as long as it is within its input common mode range) and instead the output common mode is forced by the built-in common mode amplifier with VCM_REF as its input. As shown, in Figure 23 below, the VCMO output of most differential high speed ADC’s is tied to the VCM_REF input of the LMH6555 for direct output common mode control. In some cases, the output drive capability of the ADC VCMO output may need an external buffer, as shown, to increase its current capability in order to drive the VCM_REF pin. The Electrical Characteristics Table shows the gain (Gain_VCM_REF) and the offset (VOS_CM) from the VCM_REF to the device output common mode. Figure 23. Single Ended to Differential Conversion The single ended input and output impedances of the LMH6555 I/O pins are close to 50 Ω as specified in Electrical Characteristics Table (RIN and RO). With differential input drive, the differential input impedance (RIN_DIFF) is close to 78Ω. The device nominal input common mode voltage (VI_CM) is close to 0.3V when RS1 and RS2 of Figure 23 are open. Thus, the input source will experience a DC current with 0V input. Because of this, the differential output offset voltage is influenced by the matching between RS1 and RS2. So, in a single ended input condition, if the signal source is AC coupled to one input, the undriven input needs to also be AC coupled in order to cancel the output offset voltage (VOOS). In applications where low output offset is required, it is possible to inject some current to the appropriate input (VIN + or V IN −) as an effective method of trimming the output offset voltage of the LMH6555. This is explained later in this document. The nominal value of RS1 and RS2 will also affect the insertion gain (AV_DIFF). The LMH6555 can also be used with the input AC coupled through equal valued DC blocking capacitors (C) in series with VIN + and VIN −. In this case, the coupling capacitors need to be large enough to not block the low frequency content. The lower cutoff frequency will be 1/( πREQC)Hz with REQ= RS1+ RS2 + RIN_DIFF where RIN_DIFF ≈ 78Ω. The single ended output impedance of the LMH6555 is 50 Ω. The LMH6555 Electrical Characteristics shows the device performance with 100 Ω differential output load, as would be the case if a device such as the ADC081000/ ADC081500 (single/ dual ADC) were being driven. Copyright © 2006–2011, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LMH6555 |
同様の部品番号 - LMH6555 |
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同様の説明 - LMH6555 |
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