• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 48 subscribers
  • Views 801 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Several questions about using OPA4209

Jianxiong Zou
Prodigy 150 points

Other Parts Discussed in Thread: OPA4209

I am using the OPA4209 as a voltage tracker in my design. Just input signal to the non-inverting input then connect the output and inverting input directly.

For lack of experience，I have two questions.

1. When the Rload is 50r, can the output voltage swings to 1V? I can't find the current output capability in the datasheet, only know the short current and rail-to-rail output. (It can swing to 1V into 50r in the simulation, but I want to make sure)

2. When estimate the output voltage noise of OPA4209, what bandwidth should be used? the 18MHz (the OPA4209 bandwidth,unit gain) or the bandwidth alone the signal path after the OPA4209?


  • 0
    TI__Guru 74291 points

    Jianxiong,

    The maximum output current of any op amp is only limited by the short circuit limit and thermal characteristic of the package used while the output swing is a function of the power supply voltage and output current; however, you have not provided information in this regard.  If I assume you are operating OPA4209 on dual supplies (min +/-2.25V, max +/-18V) with the 50ohm load resistor connected to the ground, this would require 20mA output current, which according to the PDS graph below would result in the output swing within 0.5V to either rail.  Therefore, OPA4209 should not have a problem to swing 1V as long as the min supply voltage is +/-2.25V.

    Be aware, however, that you need to consider thermal characteristics of the OPA4209 package to make sure you do not exceed the max 200C rated junction temperature. Since OPA4209AIPW junction-to-ambient thermal resistance is 112.9C/W (see below), driving the output to 1V with 50ohm connected to ground on +/-18V supplies would result in P=I*V = 20mA*17V=340mW power dissipation inside the package increasing the OPA4209 case temperature by Delta T = .34W*112.9 = 38C for each channel (Delta T of ~153C for four channels connected in the same way) above the ambient temperature.

    When estimating the output voltage noise in a follower configuration, you should use the unity-gain bandwidth of 18MHz *1.57 (for 1-st order low-pass AOL roll-off - see attached).  Since the 1/f noise may be neglected in this case, the Total Output Noise = 2.2nV/rt-Hz * (18e6*1.57)^0.5 = 11.7uVrms

    op-amp-noise-tech_day_small_.ppt
  • 0 in reply to Marek Lis
    Prodigy 150 points

    It is very helpful,the power supply is +/-5V.And there is only the PDS graph for +/-18V. Under the other power supply config, can the output swing be estimated from the +/-18V PDS graph? 

    By the way, the maximum capacitive load is about 1nF for stable operation (small signal), but when the input is a DC level from a DAC output, and the load capacitance is about several hundred nF, can the output be stable?

    thanks sincerely.