OPA607 datasheet(27/52 Pages) TI2

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CDIFF

VIN

VREF

VSIG

VREF

-(RF/RG)VSIG

VS+

VS-

CCM

CPCB

Figure 8-5. Inverting Amplifier With Input Capacitors

The effective capacitance at the amplifier inverting input pin is shown in Equation 5, which forms a pole in β at a

cut-off frequency of Equation 6.

DIFF

PCB

(5)

where

• CCM is the amplifier common-mode input capacitance

• CDIFF is the amplifier differential input capacitance

• CPCB is the printed circuit board (PCB) parasitic capacitance

2 R C

(6)

For low-power systems, greater the values of the feedback resistors, the earlier in frequency does the phase

margin begin to reduce and cause instability. Figure 8-6 and Figure 8-7 illustrate the loop gain magnitude and

phase plots, respectively, for the OPA810 simulation in TINA-TI configured as an inverting amplifier with values

of feedback resistors varying by orders of magnitudes.

Frequency (Hz)

-20

-10

100

110

120

100

10k

100k

10M

100M

D801

RF = 200 :, RG = 50 :

RF = 10 k:, RG = 2.5 k:

RF = 1 M:, RG = 250 k:

Figure 8-6. Loop-Gain vs Frequency for Circuit of

Figure 8-5

Frequency (Hz)

100

10k

100k

10M

100M

D802

RF = 200 :, RG = 50 :

RF = 10 k:, RG = 2.5 k:

RF = 1 M:, RG = 250 k:

Figure 8-7. Loop-Gain Phase vs Frequency for

Circuit of Figure 8-5

A lower phase margin results in peaking in the frequency response and lower bandwidth as Figure 8-8 shows,

which is synonymous with overshoot and ringing in the pulse response results. The OPA810 offers a flat-band

voltage noise density of 6.3 nV/√Hz. TI recommends selecting an RF so the voltage noise contribution does not

exceed that of the amplifier. Figure 8-9 shows the voltage noise density variation with value of resistance at

25°C. A 2-kΩ resistor exhibits a thermal noise density of 5.75 nV/√Hz which is comparable to the flat-band noise

of the OPA810. Therefore, use an RF less than 2 kΩ while still large enough to not dissipate excessive power for

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OPA810

SBOS799E – AUGUST 2019 – REVISED AUGUST 2024

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