LOG112 datasheet(12/26 Pages) TI2

12 / 26 page

OPA380

OPA2380

SBOS291G − NOVEMBER 2003 − REVISED SEPTEMBER 2007

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TRANSIMPEDANCE BANDWIDTH AND

NOISE

Limiting the gain set by RF can decrease the noise

occurring at the output of the transimpedance circuit.

However, all required gain should occur in the

transimpedance stage, since adding gain after the

transimpedance amplifier generally produces poorer

noise

performance.

The

noise

spectral

density

produced by RF increases with the square-root of RF,

whereas the signal increases linearly. Therefore,

signal-to-noise ratio is improved when all the required

gain is placed in the transimpedance stage.

Total noise increases with increased bandwidth. Limit

the circuit bandwidth to only that required. Use a

capacitor, CF, across the feedback resistor, RF, to limit

bandwidth, even if not required for stability if total output

noise is a concern.

Figure 6a shows the transimpedance circuit without any

feedback capacitor. The resulting transimpedance gain

of this circuit is shown in Figure 7. The –3dB point is

approximately 10MHz. Adding a 16pF feedback

capacitor (Figure 6b) will limit the bandwidth and result

in a –3dB point at approximately 1MHz (see Figure 7).

Output noise will be further reduced by adding a filter

(RFILTER and CFILTER) to create a second pole (Figure

6c). This second pole is placed within the feedback loop

to maintain the amplifier’s low output impedance. (If the

pole was placed outside the feedback loop, an

additional buffer would be required and would

inadvertently increase noise and dc error).

Using RDIODE to represent the equivalent diode

resistance, and CTOT for equivalent diode capacitance

plus OPA380 input capacitance, the noise zero, fZ, is

calculated by:

DIODE ) RF

DIODERF CTOT ) CF

OPA380

OUT

BIAS

F =10kΩ

(a)

STRAY = 0.2pF

F =16pF

OPA380

OUT

BIAS

F = 10kΩ

(b)

STRAY = 0.2pF

OUT

FILTER

= 796pF

FILTER

= 100

Ω

F = 21pF

OPA380

BIAS

F =10kΩ

(c)

STRAY =0.2pF

Figure 6. Transimpedance Circuit Configurations

with Varying Total and Integrated Noise Gain

(3)

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