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Low Distortion Design 4 TIPL 1324 TI Precision Labs Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites: Noise 1 3 (TIPL1311 TIPL1313)
Transcript
Page 1: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps

Presented by Collin Wells

Prepared by John Caldwell

Prerequisites: Noise 1 – 3

(TIPL1311 – TIPL1313)

Page 2: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

Distortion from Power Supplies

• Power supplies can contribute

distortion as well as noise

• Power supplies have non-zero Zout

– Output impedance of an LDO or

switching converter

• Op amp supply drops for a half-cycle

of the output waveform

– Half-wave rectified sine waves have

even harmonics:

2

Page 3: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

Demonstration – FFT of Output

3

Harmonic 0 Ohms 3 Ohms

2nd -122dB -107dB

4th -134dB -116dB

6th -147dB -121dB

8th -140dB -125dB

Page 4: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

Demonstration – FFT of Power Supply

4

Page 5: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

Distortion from Ceramic Capacitors

• Non-inverting amplifier circuit

– Gain: 3.42

– Output voltage 3.5Vrms

• 3.9nF Feedback Capacitor

– 50V Rated

– Voltage Drop: 2.48Vrms

• THD:

– X7R -68dB (Blue)

– NP0/C0G: -135dB (Red)

5

Page 6: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

Distortion From Ceramic Capacitors • Capacitance of Class II & III ceramics will

vary greatly with the applied voltage

– This is known as the “voltage coefficient of

capacitance” or VCC

• Causes:

– Dielectric constant changes with the

intensity of an applied electric field

– Capacitor dimensions change with an

applied electric field

• Reverse piezoelectric effect (ceramic

capacitors)

• Electrostatic force ( Polyester film

capacitors)

• The top graph shows a typical voltage

coefficient curve from a manufacturer’s

datasheet

• The bottom graph shows how the value of

the capacitor will change in real time for an

applied sine wave

6

70.00%

75.00%

80.00%

85.00%

90.00%

95.00%

100.00%

105.00%

0.00% 10.00% 20.00% 30.00% 40.00% 50.00% 60.00% 70.00% 80.00% 90.00% 100.00%Pe

rce

nta

ge o

f R

ate

d C

ap

ac

itan

ce

(%

)

Percentage of Rated Voltage

Example Voltage Coefficient of an X7R Capacitor

70.00%

75.00%

80.00%

85.00%

90.00%

95.00%

100.00%

105.00%

-110.00%

-90.00%

-70.00%

-50.00%

-30.00%

-10.00%

10.00%

30.00%

50.00%

70.00%

90.00%

110.00%

0 20 40 60 80 100 120 140 160 180

Pe

rce

nta

ge o

f R

ate

d C

ap

ac

itan

ce

Pe

rce

nta

ge o

f R

ate

d V

olt

ag

e

Time

Change in Capacitance for an Applied Sine Wave

Voltage Capacitance

Page 7: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

What About AC Coupling Capacitors?

• AC coupling capacitors may also produce

distortion

• Voltage across the coupling capacitor increases

as the low frequency cutoff is approached.

• Place the low frequency corner >2 decades

below desired passband.

7

1uF (6.3V) / 10kΩ

10uF (6.3V) / 10kΩ

10uF (50V) / 10kΩ

10uF (6.3V) / 100kΩ

Capacitor Shorted ININ

CCR

f2

1

Page 8: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

Distortion from Surface Mount Resistors • Thick film resistors may produce distortion

– Voltage coefficient of the resistive element

• Usually only seen for large signal voltages (>3VRMS)

– Effect is worst for small package sizes and high resistance values

8

Thick Film, 0603 Thin Film, 0603

Page 9: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

Distortion from External ESD Protection

• The capacitance of ESD diodes varies according to the equation:

• The signal across the ESD diode causes its capacitance to change

– Draws a non-linear current

– Converted to a non-linear voltage by source impedance

• Chose diodes with the lowest junction capacitance possible!

• Place them at low impedance nodes

– Non-linear current drawn does not produce a non-linear voltage

9

7.01 R

DOD

V

CC

Page 10: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

CMOS Switch Impedance

• CMOS Switches are composed of NMOS and PMOS devices in

parallel

– Allows them to conduct current in both directions

• Control voltage (VG) is typically constant

• Signal voltage (VS) is variable

• Changing VGS modulates the on-resistance of the FETs

• Varying impedance creates distortion

– Worst for high load currents / low load impedances

• Increase load impedance

• Close feedback loop around the switch

10

INPUT

CONTROL

V-

V+

NMOS

PMOS

OUTPUT

S

S

D

D

G

G

NMOSPMOS

On

-Re

sist

an

ce

Input Voltage

Combined

V- V+

“Reducing Distortion from CMOS Analog Switches” Caldwell, 1Q2015 AAJ

Page 11: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

Reducing Distortion From External Sources • Minimizing distortion from power supplies

– Chose op amps with high PSRR in the desired passband

– Design power supplies to limit their output impedance

• Bulk decoupling capacitors, linear regulators, short PCB traces

• Capacitors

– Use NP0/C0G ceramic or Polypropylene film capacitors in the signal path

– For AC coupling capacitors set fc >2 decades below the passband

• Resistors

– Use thin film surface mount resistors (Larger packages are better)

– Through-hole metal film resistors are also very good

• ESD Protection

• CMOS Switches

11

Page 12: Low Distortion Design 4 - TI Training - Low... · Low Distortion Design – 4 TIPL 1324 TI Precision Labs – Op Amps Presented by Collin Wells Prepared by John Caldwell Prerequisites:

12

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