ZHCSIM9E November   2004  – August 2018 LMH6574

PRODUCTION DATA.  

  1. 特性
  2. 应用
  3. 说明
    1.     Device Images
      1.      频率响应与 VOUT 间的关系
      2.      频率响应与增益间的关系
  4. 修订历史记录
  5. Pin Configuration and Functions
    1.     Pin Functions
    2.     Truth Table
  6. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics ±5 V
    6. 6.6 Electrical Characteristics ±3.3 V
    7. 6.7 Typical Characteristics
  7. Detailed Description
    1. 7.1 Functional Block Diagram
    2. 7.2 Feature Description
      1. 7.2.1 Video Performance
      2. 7.2.2 Feedback Resistor Selection
      3. 7.2.3 Other Applications
      4. 7.2.4 Driving Capacitive Loads
      5. 7.2.5 ESD Protection
    3. 7.3 Device Functional Modes
      1. 7.3.1 SD vs EN
  8. Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Multiplexer Expansion
  9. Power Supply Recommendations
    1. 9.1 Power Dissipation
  10. 10Layout
    1. 10.1 Layout Guidelines
  11. 11器件和文档支持
    1. 11.1 文档支持
      1. 11.1.1 相关文档
    2. 11.2 接收文档更新通知
    3. 11.3 社区资源
    4. 11.4 商标
    5. 11.5 静电放电警告
    6. 11.6 术语表
  12. 12机械、封装和可订购信息

封装选项

机械数据 (封装 | 引脚)
散热焊盘机械数据 (封装 | 引脚)
订购信息

Multiplexer Expansion

It is possible to use multiple LMH6574 devices to expand the number of inputs that can be selected for output. Figure 31 shows an 8:1 MUX using two LMH6574 devices.

LMH6574 8_1_MUX_using_2_LMH6574_SNOSC103.pngFigure 31. 8:1 MUX Using Two LMH6574 Devices

In such an application, the output settling may be longer than the LMH6574 switching specifications (~20ns), while switching between two separate LMH6574 devices. The switching time limiting factor occurs when one LMH6574 is turned off and another one is turned on, using the SD (shutdown) pin. The output settling time consists of the time needed for the first LMH6574 to enter high impedance state plus the time required for the second LMH6574 output to dissipate the left-over output charge of the first device (limited by the output current capability of the second device) and the time needed to settle to the final voltage value.

While Figure 31 MUX expansion benefits from more isolation, originating from the parasitic loading of the un-selected channels on the selected channel, afforded by individual ROUT on each multiplexer output, this configuration does not produce the fastest transition between individual LMH6574 devices. For the fastest transition, the configuration of Figure 32 can be used where the LMH6574 output pins are all shorted together.

LMH6574 alternate_8_1_MUX_faster_switching_SNCS103_v2.pngFigure 32. Alternate 8:1 MUX Expansion Schematic (for Faster SD Switching)

Figure 33 shows typical transition waveforms and shows that SD pin switching settles in less than 145 ns.

LMH6574 SD_pin_switching_waveform_output_setting_SNCS103.gifFigure 33. SD Pin Switching Waveform and Output Settling

If it is important in the end application to make sure that no two inputs are presented to the output at the same time, an optional delay block can be added, to drive the SHUTDOWN pin of each device. Figure 34 shows one possible approach to this delay circuit. The delay circuit shown will delay SHUTDOWN's H to L transitions (R1 and C1 decay) but will not delay its L to H transition. R2 should be kept small compared to R1 in order to not reduce the SHUTDOWN voltage and to produce little or no delay to SHUTDOWN.

LMH6574 20119719.gifFigure 34. Delay Circuit Implementation

With the SHUTDOWN pin putting the output stage into a high impedance state, several LMH6574’s can be tied together to form a larger input MUX. However, there is a loading effect on the active output caused by the unselected devices. The circuit in Figure 35 shows how to compensate for this effect. For the 16:1 MUX function shown in Figure 35, the gain error would be about −0.8 dB, or about 9%. In the circuit in Figure 35, resistor ratios have been adjusted to compensate for this gain error. By adjusting the gain of each multiplexer circuit the error can be reduced to the tolerance of the resistors used (1% in this example).

LMH6574 multiplexer_gain_compensation.pngFigure 35. Multiplexer Gain Compensation

NOTE

Disabling of the LMH6574 using the EN pin is not recommended for use when doing multiplexer expansion. While disabled, If the voltage between the selected input and the chip output exceeds approximately 2V the device will begin to enter a soft breakdown state. This will show up as reduced input to output isolation. The signal on the non-inverting input of the output driver amplifier will leak through to the inverting input, and then to the output through the feedback resistor. The worst case is a gain of 1 configuration where the non inverting input follows the active input buffer and (through the feedback resistor) the inverting input follows the voltage driving the output stage. The solution for this is to use shutdown mode for multiplexer expansion.