TPL0102 datasheet(29/48 Pages) TI2

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Gain

0 dB

= +

100 k

Gain

633

634

56 dB

100

158

= +

W +

(

)

33 pF

3 dB

100 k

50 kHz

W ´

100 k

Gain

6 dB

100 k

100

100 k

= +

@ +

= + =

W +

100 k

Gain

1000

1001

60 dB

~ 0

100

= +

W +

R || C

TPL0102 resistance

100

TPL0102

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SLIS134C – MARCH 2011 – REVISED SEPTEMBER 2015

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In this application, the following equations are used:

(6)

and

(7)

Where Rf and Cf are the feedback resistor and capacitor, respectively.

A 100

Ω resistor is added in series with the TPL0102 resistance in order to stop the op amp from producing

infinite gain. When the TPL0102 is at zero-scale, the resistance between terminal L and terminal W is ~0

Ω. This

would normally cause infinite gain, but with the 100

Ω resistor is series, the lowest Zin can be is 100 Ω, which at

DC will create a gain of roughly 60 dB.

(8)

Rf and Cf were chosen based upon characteristics of the potentiometer and op amp, respectively. The value of Rf

affects the level of gain, primarily at low frequencies. Since the TPL0102 has a full-scale resistance of 100 k

Ω

between terminal W and terminal L, the Rf was chosen to match this full-scale resistance, which produces the

minimum gain of 6 dB:

(9)

As frequency increases, Cf begins to have an impact on gain. A frequency roll-off will occur due to the open-loop

gain of the op amp, but in this application, the desired effect is to have Cf impact the roll off before the open loop

gain of the op amp. At a gain of 40 dB, the op amp open loop gain will force the roll off to occur at 100 kHz.

Therefore, in order for Cf to impact the roll off before the open loop gain, roll off due to the capacitor must occur

at less than 100 kHz. In this application, 50 kHz is the desired roll off frequency, resulting in a Cf value of 33 pF.

(10)

Measurements were taken with a 10 k

Ω load. A 50 Ω resistor is included at the input for termination of

measurement equipment.

8.2.1.3 Application Curves

As the TPL0102 moves from full-scale to zero-scale, Zin decreases, which causes the gain of the op amp to

increase from 6 dB to 56 dB. The amplifier does not reach the full 60 dB of calculated gain because the

resistance in the TPL0102 did not reach 0

Ω. At zero-scale, the TPL0102 had a remaining resistance of

approximately 58

Ω.

(11)

The application curve clearly shows the effect of the low pass filter created by the Rf and Cf combination. Roll off

begins as frequencies approach 50 kHz because of the pole created by the 33 pF capacitor. As the frequency

increases beyond 50 kHz, the gain decreases by -20 dB/dec until the gain levels off at 1V/V or 0dB. The gain

levels off due to the nature of non-inverting op amp transfer functions. The feedback impedance, Zf, is

approximately zero at high frequency because Cf acts as a short. As shown below, this results in a gain of 0 dB:

(12)

At approximately 3 MHz, the gain is again reduced by –20 dB/dec due to the pole created by open-loop gain of

the OPA316.

TPL0102 Datasheet(PDF) 29 Page - Texas Instruments

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