Intuitive Approach FDAs TDF Tech Day 2019 - TI Training · 2019. 9. 18. · )'$ 'liihuhqwldo...

An Intuitive Approach to Fully Differential Amplifiers

Jacob Freet

Applications Manager, High Speed Amplifiers

Texas Instruments Milwaukee Tech Day, September 17th 2019

1

Agenda

1. Background

– What is an FDA, where we see FDAs

2. Differential Input to Differential Output

– Basic operation, I/O considerations, other features

3. Single-Ended Input to Differential Output

– Operation, I/O, input considerations, tips and tricks

4. Using FDAs to the Fullest

– Circuit applications that benefit from FDA’s unique features

2

About Me

Jacob Freet –Applications Manager for High Speed Amplifiers.

–Joined TI in 2012 as a Design Engineer.

–Worked in design, systems, and applications.

–Focus on high speed op-amps, fully differential amplifiers, current-feedback amplifiers, and transimpedance applications.

–Email: [email protected]

3

BACKGROUNDSection 1

4

What Is an Fully Differential Amplifier (FDA)?

• TI defines an FDA as an amplifier

that has:

– a fully differential output

– a common mode voltage control

• Allows ± input and ± output with

only a + supply!

5

THS41xx,THS45xx,LMH65xx

Where do we see FDAs?

FDAs are used AS…

FDAs are used IN…

6

Active Baluns (single ended to differential converter)

Differential Gain Stages ADC Drivers DAC Buffers

Transimpedance Amplifiers Level Shifters Logic Converters

LiDAR Speakers Munitions Ultrasound Oscilloscopes EW

Base Stations DAQ Position Sensing DAS Machine Vision CT & PET

DIFFERENTIAL INPUT TO DIFFERENTIAL OUTPUTSection 2

7

Equivalent Circuit

• An FDA is like 2, single ended, inverting amplifiers.

• This does not cover the true function, but helps to understand basic functionality

8

How Does an FDA Really Work?

The signal enters a strong main amplifier with both a (OUT+) and a (OUT-) output

– This is a differential pair with both outputs of the pair connected to output pins.

An FDA also has a weaker common mode error amplifier

– This circuit drives the output common mode bias point of the complementary signal paths and is crucial for FDA functions.

9

VS+

IN–

+

–

High-AOLDifferential I/OAmplifier

IN+

OUT+

OUT–

+

–

+

–VCMError

AmplifierVOCM

VS+

VS–

VS–

In normal operation:Vin+ (+) = Vin- (-)

VOUT = VO+ - VO-

VOCM = (VO++VO-) / 2

VOUT / VIN = RF / RG

Basic FDA Differential Operation: DC Example

10

VO+

VO-

-1V2.5V

3.5V

1.5V

VOUT = 1.5V – 3.5V = -2V

VOCM = (1.5V + 3.5V) / 2 = 2.5V

Inputs: Outputs:

+

5V

2.5V

2.5V

2V

3V

Node voltages with respect to ground

VIN = floating, differential input

RF = 2kΩ

RG = 1kΩ

RG = 1kΩ

RF = 2kΩ

Basic FDA Differential Operation: Transient

11

VO+

VO-

VO+

VO-

VO Differential(VO+ - VO-)

VINDiff

+1V

2kΩ

2kΩ

1kΩ

1kΩ2V

-2V

How Does the Gain Work?

12

VO+

VO-

VINDiff

½ the input: VIN-

VIN+½ the input:

The Differential Input can be thought of as two “half” signals

𝑉 = 𝑉 − 𝑉𝑉 = 𝑉 − 𝑉

Can combine the halves to form a differential signal

𝑉 = −𝑉 ∗𝑅

𝑅𝑉 = −𝑉 ∗

𝑅

𝑅

Each “half” acts like an inverting amplifier

𝑉 = 𝑉 − 𝑉 ∗𝑅

𝑅

𝑉 = 𝑉 ∗𝑅

𝑅

Just like an inverting amplifier!

-

+OP1

-

+

OP2

Rx

Rx-

+

OP3

FDA Operation – A Note On Stability

For stability, we must look at the noise gain, the voltage gain from the inputs of the FDA (VIN).

13

For most high speed FDAs, the part is optimized for RF ≥ RG, a noise gain ≥ 2V/V, which for the signal is unity gain

If the customer shorts Rf, uses a Cf, or sets Rf

FDA Differential Operation – Output Swing

14

VO+

VO-

FDA datasheets will specify the maximum output swing, which refers to each output.

VS+

swing

VS-

VS+

swing

VS-

For VS+=5V and VS-=0V, the differential output of a rail-to-rail FDA can swing nearly ±5V, or10Vpp!

For example, each output of rail-to-rail FDAs can swing nearly from VS- to VS+.

Since the 2 outputs can swing the whole supply range, the differential output VOUT= (VO+) - (VO-), is 2x the max single ended range.

VS+

VS-

FDA Differential Operation - VOCM

15

VO+

VO-

VOCM = +1 V

Gain = 2V/V

VINDiff

VO+VO-

VO-

VO+


VO Common-mode(VO+ + VO-) / 2

2V

2V

-2V

1V

-1V

2kΩ

2kΩ

1kΩ

1kΩ

2V

2V

2V

-2V

FDA Differential Operation – VOCM Limitations

16

VO+

VO-

VOCM = +1 V

VO+VO-

VO-

VO+



VOCM = +1.5 V

VO+VO-

VO-

VO+



Example continued:Vin = ±1V

2V

-2V

+Vin

2V

2kΩ

2kΩ

1kΩ

1kΩ

DC Current

DC Current

0V

0V

FDA Differential Operation – Input Common Mode

17

VO+

VO-

VOCM = +1 V

-1 V

-1 V

+1 V

+1 V

𝐼 =𝑉 − 𝑉

𝑅 + 𝑅

Example: RF = RG = 1 kΩVOCM = 1 VVICM = -1 V

IDC = 1 mA

𝑉 _ =𝑉 ∗ 𝑅 + 𝑉 ∗ 𝑅

𝑅 + 𝑅

Only this node must be within

amplifier’s input voltage range

+

FDA Differential Operation – Input Common Mode

18

VO+

VO-

VOCM = +1 V

VICM = -1 V

VINDiff

VIN+

VIN-

VO+VO-

VO-

VO+


VO Common mode(VO+ + VO-) / 2

2.5V

-2.5V

2kΩ

2kΩ

1kΩ

1kΩ

SINGLE ENDED INPUT TO DIFFERENTIAL OUTPUT

Section 3

19

Basic Concepts

• Using an FDA for single to differential conversion is very similar to using a transformer (balun).

• The advantage added by an FDA is both DC coupling and wider bandwidth– the drawback is some power consumption

20

Basic FDA Single Ended to Diff: DC Example

21

VO+

VO-

0V

1V2.5V

3V

2V

VO = 2V - 3V = -1V

VOCM = (2V+3V)/2 = 2.5V

Inputs: 1V Outputs: -1V

Vin+ = Vin-VOUT = VO+ - VO-

VOCM = (VO+ + VO-) / 2

VOUT / VIN = RF / RG

VIN

5V

1.5V

1.5V

VO+

VO-RG = 1kΩ

RG = 1kΩ

RF = 1kΩ

RF = 1kΩ

Basic FDA SE-Diff: Transient

22

VO+

VO-

VO+VO-

Assume: RF = 2*RGVOCM = Mid Supply

VO-

VO+


VIN

2.5V

-2.5V

1V

-1V

2kΩ

2kΩ

1kΩ

1kΩ

1V

-1V

2V

-2V

How Does the Gain Work?

23

VO+

VO-

VIN

Differential loop keeps inputs equal, not constant; value is set by VOCM loop

Input applied to only one half of the amplifier

𝑉 =𝑉 ∗ 𝑅 +𝑉 ∗ 𝑅

𝑅 + 𝑅𝑉 =

𝑉 ∗ 𝑅

𝑅 + 𝑅

Because the inputs are driven equal, VP = VN

Solve for the two inputs

Substitute in differential output

Because the output is differential, we only see “half” the signal at each output

VN

VP

FDA SE-Diff Operation – VOCM and VOUT_CM

24

VO+

VO-

VOCM = +1 V

Assume: RF = 2*RG

VO+ VO-

VO-

VO+


VOUT_CM(VO+ + VO-) / 2

VIN

2.5V

-2.5V

1V

-1V

1V

0V

2V

2V

-2V

1V

FDA SE-Diff Operation – Input Common Mode

25

VO+

VO-

Assume: RF = 2*RGVO Differential

(VO+ - VO-)


VINVIN

VICM = -1 V1V

-1V

-2V

2V

VO+VO-

VO-

VO+

VIN

-1V

2V

4V

FDA SE-Diff Operation – Input Common Mode

26

VO+

VO-

Assume: RF = 2*RG

VO+VO-

VO-

VO+

VICM = -1 V

VICM = 0 V

The differential input to the FDA is unipolar!


VOCM is still 0V

FDA SE-Diff Operation – Balancing the Inputs

27

VO+

VO-

Assume: RF = 2*RGVOCM = 0 V

VIN

VICM = -1 V

VICM = -1 V

Set the VIN_CM to -1 V at both inputs! Set

reference to mid-signal

VO+VO-

VO-

VO+


The circuit now functions just like the differential example

-1V


Easiest way to get -1 V to balance our inputs?

28

VICM = -1 V

VO+

VO-

How about a resistor divider?

This will NOT work so easily.

Current flows through the noninverting RG resistor that will change the effective voltage at the input to RG.

Another way to think about it is that the AC impedances should balance. RG ≠ RG + R||R

current

2V

-2V


Easiest way to get -1 V to balance our inputs?

29

VICM = -1 V

VO+

VO-

Use a resistor divider with the right Thevinin Equivalent

2V

-2V

= 2RG

= 2RG

-2V

The ratio should set the reference voltage to mid-signal (-1V) and the parallel impedance should be RG.

Use the same idea on signal side if attenuation is required! (ex. HV Power Quality)


Alternate solution?

30

VICM = -1 V

VO+

VO-

Use a buffer amplifier if required resistor values are unavailable

Why not a regulator?

The source needs to have a constant impedance over the FDA bandwidth.

V = -1 V

Same idea can be used on signal side if high-Z input is required!

FDA SE-Diff Operation – Other Input Issues

31

For HS transmission line applications, the signal has source impedance (RS). (50Ω, 75Ω, etc.)

If we just tie the non-signal RG to ground, it will cause a gain offset and distortion because the gain impedances are imbalanced.

The simple solution?

Just terminate the non-signal RG with the same impedance as the input source. Alternatively you can reduce the signal RG.


What about the input impedance? Is it not simply RG?

32

ZIN

Not quite. The input impedance looks like RGterminated to the input common mode of the amplifier.

𝑉 =𝑉 ∗ 𝑅 + 0.5 ∗ 𝑉 ∗ 𝑅

𝑅 + 𝑅

However, for SE to Diff, the input common mode changes with the input voltage!

This causes the input impedance to be terminated to a varying voltage!

VICMAMP


Because the input impedance is terminated to a changing voltage, we call it an “active impedance”.

33

ZIN

Why do we care?

The input impedance will be constant as long as the the VICM value can move with the input, but that is limited by the weaker VOCM amplifier’s bandwidth

As the frequency increases, the loop gain will decrease and eventually the input will not be a good match.

VICMAMP

*larger number is better match

USING FDAs TO THE FULLESTSection 3

34

FDA Summary and Inherent Advantages

1. Single ended signal to differential load with DC coupling

2. Maintains the output common mode voltage for a wide input signal range

3. Improved HD2, EMI, and noise performance compared to single ended amplifier

4. More symmetrical and more power efficient than equivalent function with two discrete amplifiers.

35

Using FDAs for Level Shifting

• Can use one or more FDAs to level shift a signal using the VOCM pin.

• May require different supply voltages depending on how much of a shift is required.

• TI Design: TIDA-00659

36

Using an FDA for High Side Current Sense

• Can use the benefit of the FDA input to output common mode to allow for input signals larger than the maximum input.

• Useful for applications such as high side current sensing.

• TI Design: TIDA-00976

37

Quiz:

How can customer improve SNR?*Assume Gain = 2 V/V

38

VO+

VO-

VIN

VO-

VO+

VO+

VO-


This is not ideal, we are throwing away the positive

half of available output range!

What can be done?

1V

-1V

2V

-2V

Maximize FDA Dynamic Range

What can be done?1. Apply a voltage equal to mid

signal to the noninverting input (Vref).

2. Double the gain.

39

VO+

VO-

VIN

VO-

VO+

VO+VO-


The dynamic range is now double and using the full

differential output!

V= 0.5V

Low-Z

2

2

2V

-2V

1V

-1V

Intuitive Approach FDAs TDF Tech Day 2019 - TI Training · 2019. 9. 18. · )'$ 'liihuhqwldo...

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Transcript of Intuitive Approach FDAs TDF Tech Day 2019 - TI Training · 2019. 9. 18. · )'$ 'liihuhqwldo...