Texas Instruments | Circuit for differential output from a single-ended precision DAC | Application notes | Texas Instruments Circuit for differential output from a single-ended precision DAC Application notes

Texas Instruments Circuit for differential output from a single-ended precision DAC Application notes
Analog Engineer's Circuit: Data
Converters
SBAA344 – January 2019
Circuit for differential output from a single-ended
precision DAC
Uttama Kumar Sahu
Design Goals
Power Supply
DAC Output
Offset Range
VCC: 10V, VSS: –10V, VDD: 5V
0V to 5V
±5V
Design Description
This single-ended to differential conversion circuit features two operational amplifiers (op amp). It
generates a bipolar differential output from a unipolar digital-to-analog converter (DAC) channel. These
types of circuits are very useful in low-noise applications like Optical module, Metro data center
interconnect, Ultrasound scanners, and X-ray systems. While another design approach using a fullydifferential amplifier (FDA) is good for AC performance, the op amp approach will achieve best DC
characteristics. Nevertheless, the specific op amp or FDA will impact the comparison of the two
topologies.
VDD
±
VREF
DAC
VDAC
VOUTP
U1
+
R1
R2
±
U2
+
VDIFF
VOUTN
VCM
Design Notes
1. Choose a DAC with required resolution and output range
2. Select an op amp to meet the system specifications considering the following key requirements:
• Swing to rail: For 5-V supply rails it is common to use a rail-to-rail zero-crossover distortion device
(for example, OPA320 and OPA365)
• Offset voltage and drift: One of the advantages of this circuit over the FDA approach is that some
op amps can have very good DC performance
• Bandwidth and quiescent current: Another advantage of this circuit over the FDA approach is that
a wide range of op-amp bandwidth and related quiescent currents are available. For lower
sampling rate, a low-bandwidth, low-current op amp may be an optimal choice
3. Choose R1 and R2 to minimize thermal noise at the output
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Design Steps
1. Select a DAC such as DAC80501 that is a 16-bit single channel buffered voltage output DAC with
2.5-V internal reference. The reference output can also be used as the common-mode voltage (VCM)
2. Select a low-distortion op amp such as OPA320
3. The DC transfer function of the circuit is described with the following equations:
•
•
VOUTP
VDAC
VOUTN
§ R2 ·
VCM ¨ 1
¸ VDAC
R1 ¹
©
R1 and R2 is chosen as 1kΩ to have a gain of 1 and to minimize noise. Use 0.1% tolerance to
minimize gain error.
The thermal noise contributed by U1 to the differential output has two paths: directly through U1
and with inversion through U2. Both these noise are correlated and hence they will add directly.
DAC80501 has an output noise density (en-DAC) of 74nV/√Hz and OPA320 has a noise density (enAMP) of 7nV/√Hz. The noise gain of U1 (Gn-U1) is 1. So, the total noise density contributed by output
of U1 (en-U1) is given by:
en
•
en
2
en
DAC
2
AMP
148.66nV
Hz
R
4.K.T.(R1 || R2 )
4.(1.38 u 10
23
J / K).(298.15K).(500:)
2.87nV
Hz
The uncorrelated noise density contributed by U2 (en-U2) is a combination of the thermal noise of the
gain resistors (en-R), thermal noise of U2 (en-AMP), and the noise contributed by VCM when fed through
the VREF output of DAC80501 (en-VREF). en-VREF is 140nV/√Hz. The noise gain of U2 (Gn-U2), that is,
1+(R2/R1) is 2. So, en-U2 is written as:
en
•
2u
The thermal noise contributed by the gain resistors R1 and R2, (en-R) is given by:
en
•
U1
U2
en
VREF .Gn U2
2
en
AMP .Gn U2
2
en R .Gn
2
U2
280.4nV
Hz
Finally, combining the noise from U1 and U2, we get the total noise density at the differential output
(en-T):
en
T
en
2
U1
en
2
U2
317.37nV
Hz
The simulated value for the thermal noise at the differential output is shown in the following figure. The
simulated value 316.9nV/√Hz is close to the calculated value. The thermal noise for the DAC output
and VREF output were emulated with equivalent resistors for noise simulation.
2
Circuit for differential output from a single-ended precision DAC
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Thermal Noise Density at The Differential Output (VCM = VREF)
600.00n
T
10kHz, 316.9nV/rtHz
VDIFF
300.00n
500.00n
VOUTN
200.00n
200.00n
VOUTP
0.00
1.00
1.00k
Frequency (Hz)
1.00MEG
DC Transfer Characteristics (VCM = 0V)
T
20.00
10V
VDIFF
0.00
1.00
VOUTN
0V
-5.00
6.00
VOUTP
5V
0.00
0.00
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2.50
V-DAC (V)
5.00
Circuit for differential output from a single-ended precision DAC
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Frequency Response (Amplitude)
10.00
T
VDIFF
-10.00
10.00
VOUTN
-40.00
3.00
VOUTP
-9.00
10.00
31.62k
Frequency (Hz)
100.00MEG
31.62k
Frequency (Hz)
100.00MEG
Frequency Response (Phase)
0.00
T
VDIFF
-300.00
200.00
VOUTN
-300.00
0.00
VOUTP
-300.00
10.00
4
Circuit for differential output from a single-ended precision DAC
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Group Delay
Group delay is the time delay between the applied input signal and the output signal. All amplifiers and
filters have a group delay. Group delay is highlighted for this circuit because the inverting and noninverting
path both have different group delays. This can create distortion for higher frequency signals. See the
group delay in time domain plot for additional detail.
T 30.00n
VOUTN
Time (s)
20.00n
10.00n
VOUTP
0.00
10.00
31.62k
Frequency (Hz)
100.00MEG
Output Transient Response
5.00
T
V-DAC
0.00
20.00
VDIFF
-10.00
1.00
VOUTN
-6.00
6.00
VOUTP
-1.00
0.00
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10.00u
Time (s)
20.00u
Circuit for differential output from a single-ended precision DAC
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Design Featured Devices and Alternative Parts
Device
Key Features
Link
DAC80501
16-bit, 1-LSB INL, digital-to-analog converter (DAC) with precision internal reference
http://www.ti.com/product/DAC80501
DAC80508
8-channel, true 16-bit, SPI, voltage-output DAC with precision internal reference
http://www.ti.com/product/DAC80508
DAC8562
16-bit, dual-channel, low-power, ultra-low glitch, voltage output DAC with 2.5V, 4ppm/°C
reference
http://www.ti.com/product/DAC8562
OPA320
Precision, zero-crossover, 20MHz, 0.9pA Ib, RRIO, CMOS operational amplifier
http://www.ti.com/product/OPA320
OPA365
2.2-V, 50-MHz, low-noise, single-supply rail-to-rail operational amplifier
http://www.ti.com/product/OPA365
Design References
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
Link to Key Files
TINA source files – http://www.ti.com/lit/zip/sbam419.
For direct support from TI Engineers use the E2E community:
e2e.ti.com
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Circuit for differential output from a single-ended precision DAC
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