OPA643P datasheet(9/18 Pages) TI1

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OPA643

The input signal and the gain resistor are AC coupled

through the 0.1

µF blocking capacitors. This holds the DC

input and output operating point at ground independent of

source impedance and gain setting. The R

G value shown in

Figure 2 (144

Ω) sets the gain to the matched load at 12dB.

Using standard 1% tolerance resistors for R

F and RG will

hold the gain to a

±0.2dB tolerance. This example will give

a –3dB bandwidth of approximately 100MHz while

maintaining gain flatness within 1dB through 50MHz. For

narrowband IF’s in the 21.4MHz region, this configuration

of the OPA643 will show a third-order intercept of 40dBm

while dissipating only 200mW (23dBm) power from

±5V

supplies.

PHOTODIODE TRANSIMPEDANCE AMPLIFIER

High Gain Bandwidth Product (GBP) and low input voltage

and current noise make the OPA643 an ideal wideband

transimpedance amplifier for low to moderate gains. Note

that unity gain stability is not required for application as a

transimpedance amplifier. Figure 3 shows an example

photodiode amplifier circuit. The key parameters of this

design are the estimated diode capacitance (CD) at the

applied DC reverse bias voltage (–V

B), the desired

transimpedance gain (RF), and the GBP for the OPA643

(800MHz). With these three variables set (and adding the

OPA643’s parasitic input capacitance to the value of CD to

get C

S), the feedback capacitor value (CF) may be chosen to

control the transimpedance frequency response.

pin. This capacitor provides a low source impedance for the

transient currents produced by the sampling process.

Improved SFDR is obtained by adding the capacitor, whose

value is often recommended in the converter data sheet. The

external capacitor, in combination with the built-in

capacitance of the A/D input, presents a significant capacitive

load to the OPA643. Without a series isolation resistor, the

result can be peaking and possibly oscillation in the amplifier.

Refer to the plot of “RS vs Capacitive Load” in the Typical

Performance Curves to obtain a good starting value for the

series resistor. The values shown in this curve will ensure a

flat frequency response at the input of the ADC. Increasing

the external capacitor value will allow either the series

resistor to be reduced, or, keeping this resistor fixed, will

bandlimit the signal and reduce high frequency noise to the

input of the converter.

WIDE DYNAMIC RANGE IF AMPLIFIER

The OPA643 offers an attractive alternative to standard

fixed gain IF amplifier stages. Narrowband systems will

benefit from the exceptionally high two tone third-order

intermodulation intercept as shown in the Typical

Performance Curves. Op amps with high open-loop gain,

like the OPA643, provide an intercept that decreases with

frequency along with the loop gain. The OPA643’s intercept

is > 25dBm up to 50MHz but improves to > 50dBm as the

operating frequency is reduced below 10MHz. Broadband

systems will also benefit from the very low even order

harmonics and intermodulation components produced by the

OPA643. Compared to standard fixed gain IF amplifiers, the

OPA643 operating at IF’s below 50MHz provides much

higher intercepts for its quiescent power dissipation (200mW),

superior gain accuracy, higher reverse isolation, and lower

I/O return loss. Noise figure for the OPA643 will be higher

than alternative fixed gain stages. If the application comes

late in the amplifier chain with significant gain in prior

stages, this higher noise figure will be acceptable. Figure 2

shows an example non-inverting configuration for the

OPA643 used as an IF amplifier.

10k

Ω

Supply Decoupling

Not Shown

20pF

OPA643

+5V

–5V

–V

O = ID RF

0.8pF

FIGURE 3. Wideband, Low Noise, Transimpedance

Amplifier.

To achieve a maximally flat second-order Butterworth

frequency response, the feedback pole should be set to:

1/(2

πR

FCF) = √(GBP/(4πRFCS))

Adding the OPA643’s common-mode and differential mode

input capacitances (1.3 + 2.5)pF to the 20pF diode source

capacitance of Figure 3, and targeting a 10k

Ω transimpedance

gain using the 800MHz GBP for the OPA643, the required

feedback pole frequency is 16.4MHz. This will require a

total feedback capacitance of 1.0pF. Typical surface mount

resistors have a parasitic capacitance of 0.2pF, leaving the

Ω

Gain =

= 20log 1/2 1+

= 12dB with values shown

Supply Decoupling

Not Shown

Ω

144

Ω

52.3

Ω

0.1µF

OPA643

+5V

–5V

Ω Load

(

)

0.1µF

Ω Source

FIGURE 2. Wide Dynamic Range IF Amplifier.

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