CLC412 datasheet(12/20 Pages) TI1

12 / 20 page

OBSOLETE

CLC412

SNOS833D – AUGUST 2000 – REVISED APRIL 2013

www.ti.com

Figure 36. Rf vs. Inverting Gain (AJE & AJP)

Both plots show the value of Rf approaching a non-zero minimum (dashed line) at high gains, which is

characteristic of current-feedback op amps, while the linear portion of the two (solid) curves (i.e., -5 > AV > +6)

results from the limitation placed on Rg (i.e., Rg ≥ 50Ω). This limitation is due to the desire to keep Rg greater in

value than that of the inverting input resistance. Therefore, the resulting small-signal bandwidth curves, labeled

“BW”, correspond to the two (solid) “Rf” curves. These results may deviate from that produced by the analysis of

OA-13 since these plots were produced from an actual board layout that included parasitic capacitances not

accounted for by the analysis of OA-13. It should be noted that a non-inverting gain of +1V/V requires an Rf =

Ω and the output voltage used for both plots is 2VPP.

In order to bandlimit the CLC412 at any particular gain setting, a larger value of Rf (than previously

recommended in the plots above) is needed. Following the analysis in OA-13, we find the CLC412's “optimum

feedback transimpedance”, Zt*, below.

(1)

The “optimum feedback transimpedance” is unique for each current-feedback op amp and determines the

recommended value of Rf for a particular gain setting. Drawing a horizontal line on the “Open-loop

Transimpedance, Z(s)” plot from 57.5dB (on the left vertical axis), we find the intersection with the

transimpedance magnitude trace occurs at a frequency of 180MHz. This frequency is only an approximation of

the CLC412's small-signal bandwidth. From this intersection, one can see that an increase in Zt will produce a

new intersection occurring at a lower frequency. This is the process to follow when bandlimiting. Once the target

small-signal bandwidth is determined, the new value of Zt is picked off the graph at the point where this

frequency and the transimpedance magnitude trace intersect. One can then back track to figure the value of the

feedback resistor, Rf = Zt − Rin (1 + Rf/Rg). This new value of Rf will produce the desired frequency roll-off.

Submit Documentation Feedback

Product Folder Links: CLC412

CLC412 Datasheet(PDF) 12 Page - Texas Instruments