Texas Instruments | Current-to-voltage converter circuit for audio DACs | Application notes | Texas Instruments Current-to-voltage converter circuit for audio DACs Application notes

Texas Instruments Current-to-voltage converter circuit for audio DACs Application notes
Analog Engineer's Circuit: Data
Converters
SBAA333 – January 2019
Current-to-voltage converter circuit for audio DACs
Paul Frost
Design Goals
DAC IOUT Amplitude
DAC IOUT Common Mode
Current
VOUT Amplitude
7.8 mAP-P
–6.2mA
2.1VRMS
Design Description
Many high-performance audio digital-to-analog converters (DACs) feature a current output that must be
converted into a voltage output for use with audio amplifiers. In premium automotive audio applications,
for example, it is critical to ensure the DAC signal-to-noise ratio (SNR) and total harmonic distortion plus
noise (THD+N) performance is not compromised by the current-to-voltage (I-V) output stage.
R1 820
C1 4.7nF
R3 240
PCM1794A-Q1
-10V
±
+
+
OUTP
C2 10nF
R2 511
OPA1612
-5V
+5V
±
+5V
+
OUTN
+
OPA1612
+
DSM
+
511
OPA1612
+5V
±
-10V
10nF
240
4.7nF
820
Design Notes
1. The first stage of the circuit converts the current output to a voltage output by providing a virtual
ground potential to the OUTP and OUTN nodes of the DAC. The current flowing from the DAC then
creates a voltage across the resistor, R1, in the feedback loop. Note that current-output audio DACs
have a common-mode current that creates an offset to ensure that the DAC is always sourcing current.
This common-mode current also results in the output of the first stage always being a negative value.
The supplies of the amplifiers of the circuit are not required to be symmetrical.
2. The second stage of the circuit is a differential amplifier that converts the differential voltage of the first
stage into a single-ended output voltage.
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Design Steps
1. The DAC is selected based on the needs of the application. Consider the required SNR, THD+N, and
supported I2S interface sample rates. While most audio DACs support rates ranging from 16kHz to
192kHz, not all support rates like 384kHz or 768kHz. Higher rates result in noise shaping that moves
the out-of-band noise further from the audible range, but not all audio sources can provide them. Also
note that not all current-output audio DACs have the same amplitude and common-mode current.
2. Select the amplifier based on its noise and THD performance. Ideally, these amplifiers should not gate
the SNR performance of the DAC. A JFET or bipolar-input amplifier is recommended for its low
voltage-noise. The higher current-noise is not an issue as the resistance values of the circuit are low.
3. The gain of the circuit is calculated with the following equation. Using R1 = 820Ω, R2 = 511Ω, and R3
= 240Ω, the output voltage will be approximately 6VP-P or 2.1VRMS. For this circuit, superior noise
performance is achieved by having the first stage have a large gain, and the second stage actually
attenuate the signal.
VOUT (IIN )
§ R3 ·
IIN u R1u ¨
¸
© R2 ¹
4. The cutoff frequency (fC) of the first stage can be calculated with the following equation:
fC -FIRSTSTAGE =
1
2 ´ p ´ R1 ´ C1
The fC of the second stage is calculated with the following equation:
fC - SECONDSTAGE =
1
2 ´ p ´ R3 ´ C2
Using C1 = 4.7nF and C2 = 10nF, the fC of the first stage is approximately 44.2kHz and the fC of the
second stage is approximately 66.3kHz.
5. The capacitors used for the filter should be COG/NP0 type ceramics. COG/NP0 type capacitors have a
lower voltage coefficient of capacitance, meaning that the capacitive value of the component is less
impacted by the voltage bias across the device. As the capacitors are key for performance of the filter,
other types of ceramic capacitors should be avoided in the signal path.
6. Thin-film resistors are recommended for the resistive elements in the circuits. All resistors feature
voltage noise, which is well understood to be dependent on resistance and temperature, as shown in
the first equation that follows. But resistors also have a current-noise, which is dependent on the
voltage across the resistor, frequency, and a constant, C, that is dependant on the material of which
the resistor is composed, as shown in the second following equation:
ST = 4kRT,
where
• k is Boltzman's constant
• R is resistance
• T is temperature
SE = ( C × U2 ) / ƒ
where
•
•
•
2
C is a constant derived from the resistor material
U is the differential voltage across the resistor
ƒ is the frequency
Current-to-voltage converter circuit for audio DACs
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DC Transfer Characteristics
The simulation shows that the 7.8-mAP-P differential input current results in an approximately 6VP-P output,
or 2.1VRMS.
Filter Characteristics
The filters created by the two amplifier stages have a –3-dB corner frequency of approximately 32.1kHz.
This will attenuate out-of-band noise from the output, while not affecting the audible range (20Hz to
22kHz).
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Noise Simulation
The following simulation shows the benefit of having the majority of the gain of the circuit in the first stage.
The two curves were generated by the same circuit with different resistor values, but resulting in the same
total gain. The green line has a higher gain in the first stage, but lower total noise.
4
Current-to-voltage converter circuit for audio DACs
Copyright © 2019, Texas Instruments Incorporated
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Design References
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
See the HiFi Audio circuit design application report.
Download the companion simulation files for this circuit.
For direct support from TI Engineers use the E2E community
E2E Community
Design Featured Devices
Device
Key Features
Link
Other Possible Devices
PCM1794A-Q1
24-bit, 192-kHz sampling, advanced segment, audio
stereo digital-to-analog converter
http://www.ti.com/product/PCM1794A-Q1
Audio DACs from Texas Instruments
OPA1612
Ultra-low noise, ultra-low distortion, high-performance,
bipolar-input audio operational amplifier
http://www.ti.com/product/OPA1612
Audio Amplifiers from Texas
Instruments
Other Links:
Learn more about how to use precision DACs at our Precision DAC Learning Center.
Learn about TI's audio DAC portfolio and find more technical content.
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