OPA1644AIPWR datasheet(21/50 Pages) TI2

21 / 50 page

OPA1641

SignalGain=1+

DistortionGain=1+

V =3V

RMS

Generator

Output

Analyzer

Input

AudioPrecision

SystemTwo

(1)

withPCController

SIGNAL

GAIN

DISTORTION

GAIN

100W

1kW

10W

11W

101

R IIR

Load

OPA1641, OPA1642, OPA1644

www.ti.com

SBOS484D – DECEMBER 2009 – REVISED APRIL 2016

Product Folder Links: OPA1641 OPA1642 OPA1644

Submit Documentation Feedback

8.1.3 Total Harmonic Distortion Measurements

The OPA164x series operational amplifiers have excellent distortion characteristics. THD + noise is below

0.00005% (G = 1, VO = 3 VRMS, BW = 80 kHz) throughout the audio frequency range, 20 Hz to 20 kHz, with a

2-k

Ω load (see Figure 7).

The distortion produced by the OPA164x series operational amplifiers is below the measurement limit of many

commercially available distortion analyzers. However, a special test circuit (such as shown in Figure 35) can be

used to extend the measurement capabilities.

Operational amplifier distortion can be considered an internal error source that can be referred to the input.

Figure 35 shows a circuit that causes the operational amplifier distortion to be 101 times (or approximately

40 dB) greater than that normally produced by the operational amplifier. The addition of R3 to the otherwise

standard noninverting amplifier configuration alters the feedback factor or noise gain of the circuit. The closed-

loop gain is unchanged, but the feedback available for error correction is reduced by a factor of 101, thus

extending the resolution by 101. Note that the input signal and load applied to the operational amplifier are the

same as with conventional feedback without R3. Keep the value of R3 small to minimize any effect on distortion

measurements.

The validity of this technique can be verified by duplicating measurements at high gain or high frequency where

the distortion is within the measurement capability of the test equipment. Measurements for this document were

made with an audio precision system two distortion and noise analyzer that greatly simplifies such repetitive

measurements. The measurement technique can, however, be performed with manual distortion measurement

instruments.

space

(1)

For measurement bandwidth, see Figure 7 through Figure 10.

Figure 35. Distortion Test Circuit

8.1.4 Source Impedance and Distortion

In traditional JFET-input operational amplifiers, the impedance applied to the positive and negative inputs in

noninverting applications must be matched for lowest distortion. Legacy methods for fabricating the JFETs in the

FET input stage exhibit a varying input capacitance with applied common-mode input voltage. In inverting

configurations, the input does not vary with input voltage because the inverting input is held at virtual ground.

However, in noninverting applications, the inputs do vary, and the gate-to-source voltage is not constant. This

effect produces increased distortion as a result of the varying capacitance for unmatched source impedances.

However, the OPA164x family of amplifiers is designed to maintain a constant input capacitance with varying

common-mode voltage to prevent this mechanism of distortion. The variation of input capacitance with common-

mode voltage for a traditional amplifier is compared to the OPA164x family in Figure 36.

OPA1644AIPWR Datasheet(PDF) 21 Page - Texas Instruments

OPA1644AIPWR Datasheet(HTML) 21 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Datasheet Download

Link URL