1
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Development of a Low -Cost Programmable Microphone Preamp
Gain Control IC for Pro Audio Applications
Gary Hebert, Chief Technology Officer
THAT Corporation
2
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Tonight’s Presentation
• Introduction• Professional Microphone Preamplifiers• Digital Mic Preamp Gain Controllers• Earlier Products• Cost Reduction Measures• Cost-Performance Tradeoffs• Measured Performance• Conclusions
3
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Who’s THAT?
• Founded in 1989– 2014 was our 25th anniversary!
• Spin-off from dbx Inc.• Founders were dbx engineers
– Paul Travaline, Gary Hebert, And Les Tyler• Once made complete pro-audio
products• Now focused on Pro Audio ICs and
Licensing
4
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Professional Microphone Preamps
• Balanced (Differential) Input• Low input noise required
– On the order of 150Ω thermal noise– (-130.8 dBu in 20 Hz – 20 kHz BW)
• Wide gain range required– Mic sensitivities vary over at least 37 dB– Sound levels vary with application
• Max input level should be ≥ +16 dBu for the highest-output condenser microphones
5
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Digital Control of Professional Microphone Preamps
• Many preamps are now front ends for A/D converters in digital audio products.
• Digital control of the gain gives a uniform user interface for these systems.
• It also allows enhanced automation features such as setup recall and automatic gain reduction in response to clipping.
6
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Typical Preamp Front End
• Differential gain = 1+ (2RF/RG)
• CC capacitors block dc inputs and phantom power
• RG low valued at high gains to minimize noise
7
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
What’s a Programmable Gain Controller?
• Digitally controlled feedback network for a low-noise differential amplifier
8
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Programmable Gain Controller
• RG gets small at high gains
• Small RG implies low RON switches
• RF/RG is large at high gains
9
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
THAT’s First Controllers
• 5171 - 1 dB/step– 13.6 dB – 68.6 dB– <.0008% THD, +24 dBu out, any gain
• 5173 - 3 dB/step– 0 dB – 60 dB– <.001% THD,+24 dBu out, any gain
• Accurate gains - +/-.5 dB max, +/-.15 dB typical
• DC servos
10
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
5171/5173 Topology
• Both RF and RG are varied using a combination of a tapped resistor string and a set of switched paralleling resistors.
11
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
5171/5173 Topology
• Tapped resistor string is used for large steps
• Tapped string switches don’t effect gain or THD, but do add noise
12
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
5171/5173 Topology
• Minimum RG (in red) is 5.6Ω in the 5171.
• This resistor is very wide and short.
• W/L≈109 for 610 Ω/sq. poly
13
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
5171/5173 Topology
• Paralleling resistors are used for small steps
• Paralleling switches are in series with high-value resistors
14
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
5171/5173 Topology
• Maximum parallel RF (in red) is 47 kΩ in the 5171.
• This resistor is narrow and long.
• W/L≈.013 for 610 Ω/sq. poly
15
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Show Me the Money
• The 5171 and 5173 have proven to be too expensive for many applications, particularly those at the entry level where some of the possible automation features might be most useful.
• So, what makes them expensive, and what do we trade off to make a less costly part?
16
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Resistors – Bigger is Better
• Ratio accuracy increases with resistor area.
• Distortion due to self heating is proportional to: ∗
.
17
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Switches – Bigger is Better
• RON is inversely proportional to device width
• Low RON minimizes noise from the tap-string switches
• High voltage capability also increases area
18
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Cost Reduction – the Easy Stuff
• 3 dB per step• Dual channel part
– Saves some package cost– Small savings in SPI interface area
• Eliminate servo– Reduces die area– Reduces power– Requires large external capacitor
19
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
New Topology
• Variable RG
• Fixed RF
• Switch RON added to RG
• ∆RON adds THD• RON variation
adds gain error
20
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
New Topology
• Dynamic gate drive minimizes THD due to ∆RON
• Reduced max gain (51 dB) saves die area
21
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
How Much Gain?
85
95
105
115
125
-20 -10 0 10 20 30 40 50
Dy
na
mic
Ra
ng
e (
dB
)D
yn
am
ic R
an
ge
(d
B)
Dy
na
mic
Ra
ng
e (
dB
)D
yn
am
ic R
an
ge
(d
B)
Gain (dB)Gain (dB)Gain (dB)Gain (dB)
Dynamic Range vs. Gain Dynamic Range vs. Gain Dynamic Range vs. Gain Dynamic Range vs. Gain –––– 150150150150ΩΩΩΩ Source, Ideal Source, Ideal Source, Ideal Source, Ideal PreampPreampPreampPreamp
114 DB A/D DynamicRange (dB)
120 dB A/D DynamicRange (dB)
22
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
“Bent Binary” R G Scheme
• Binary resistances for RG leads to gain error at low gains
• Bending a few of the LSBs gives a good fit (+/-.2 dB nominal error)
-1.00-1.00-1.00-1.00
-0.80-0.80-0.80-0.80
-0.60-0.60-0.60-0.60
-0.40-0.40-0.40-0.40
-0.20-0.20-0.20-0.20
0.000.000.000.00
0.200.200.200.20
0.400.400.400.40
0.600.600.600.60
0.800.800.800.80
1.001.001.001.00
0000 3333 6666 9999 12121212 15151515 18181818 21212121 24242424 27272727 30303030 33333333 36363636 39393939 42424242 45454545 48484848 51515151
Ga
in E
rro
r (d
B)
Ga
in E
rro
r (d
B)
Ga
in E
rro
r (d
B)
Ga
in E
rro
r (d
B)
Nominal Gain (dB)Nominal Gain (dB)Nominal Gain (dB)Nominal Gain (dB)
Gain Error vs. Nominal GainGain Error vs. Nominal GainGain Error vs. Nominal GainGain Error vs. Nominal Gain
Binary R's
Bent R's
23
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
“Bent Binary” R G Scheme
• 0 – 51 dB gain range with 10 switches
• Actual gain accuracy will vary since RONdoesn’t track poly resistors -1.00-1.00-1.00-1.00
-0.80-0.80-0.80-0.80
-0.60-0.60-0.60-0.60
-0.40-0.40-0.40-0.40
-0.20-0.20-0.20-0.20
0.000.000.000.00
0.200.200.200.20
0.400.400.400.40
0.600.600.600.60
0.800.800.800.80
1.001.001.001.00
0000 3333 6666 9999 12121212 15151515 18181818 21212121 24242424 27272727 30303030 33333333 36363636 39393939 42424242 45454545 48484848 51515151
Ga
in E
rro
r (d
B)
Ga
in E
rro
r (d
B)
Ga
in E
rro
r (d
B)
Ga
in E
rro
r (d
B)
Nominal Gain (dB)Nominal Gain (dB)Nominal Gain (dB)Nominal Gain (dB)
Gain Error vs. Nominal GainGain Error vs. Nominal GainGain Error vs. Nominal GainGain Error vs. Nominal Gain
Binary R's
Bent R's
24
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Resistor Area Reduction
• Resistors scaled down to meet the target die area
• These become the dominant distortion mechanism
• THD due to resistor self heating is almost pure 3rd harmonic
25
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Performance
• EIN with THAT 1580 = -128.3 dBu with 150Ω RS, 20 Hz – 20 kHz BW, 51 dB gain
• Gain Accuracy– +/-.5 dB 0 - 39 dB– +/-1 dB 42 – 51 dB
26
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Performance
0.00010
0.00100
0.01000
0.10000
1.00000
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51
THD+N vs Gain at 24dBu Out, 1 kHz
27
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Performance
28
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Performance
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51
Typical vs. Theoretical Gain ErrorTypical vs. Theoretical Gain ErrorTypical vs. Theoretical Gain ErrorTypical vs. Theoretical Gain Error
Typical Gain Error Theoretical Gain Error
29
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Additional Features
• Zero-crossing detectors (ZCDs) for each channel
• Internal time-out clock counter for ZCD• 1 general purpose logic output (GPO) per
channel• GPOs can be sync’d to the ZCD• Independent connections for RF resistors for
discrete preamp designs that require this
30
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Conclusions
• We achieved a 55% cost reduction per channel compared to our 5173
• THD performance was compromised in a manner that seems acceptable to most
• Noise performance is actually slightly better than the previous designs at most gains
31
Development of a Low-Cost Programmable Mic Preamp Gain Controller ICGreater Northwest AES Section, June2016
Copyright © 2016, THAT Corporation
Acknowledgements
Thanks to Fred Floru of THAT Corporation for his help in reviewing this presentation.
Thanks to Rene Jaeger for inviting me to speak tonight.
Top Related