TPS7A39
ZHCSGP0A –JULY 2017–REVISED SEPTEMBER 2017
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8.2.2.1 Design Requirements
A common problem in analog-to-digital converters (ADCs) is that as the input signal approaches the edge of the
range of the ADC, the signal begins to become distorted. Often times this is not because of a limitation of the
ADC, but is a result of the analog front-end (AFE). In the AFE, the signal begins to approach the rails of the op
amp and the signal begins to lose linearity and becomes distorted. This distortion is because when the rail-to-rail
op amp begins to enter the nonlinear region of operation within 100 mV of the rail, the signal-to-noise ratio (SNR)
starts to degrade and the total harmonic distortion (THD) of the ADC increases. To prevent the op amp from
exiting the linear region of operation, the design must use a power supply that can generate rails 200 mV above
and below the input range of the ADC.
8.2.2.2 Detailed Design Procedure
In this design, the ADS8900B is used as the ADC. This ADC features a differential input, so from a 5-V reference
the ADC is able to encode values between ±5 V. In many applications, single-supply op amps are powered with
rails from 0 V to 5 V, which causes the input signal to become distorted when the full range signal is applied. The
FFT of a 10-VPP (peak-to-peak) sine wave using a single 5-V rail to bias the amplifiers is illustrated in 图 79. In
this test the SNR was calculated to be 54.89 dB and the THD was calculated to be –40.68 dB.
There is a simple solution to improve the SNR and THD of the ADC: bias the amplifiers in the analog front end
with a 5.2-V rail and a –0.2-V rail. Using these rails allows the amplifier to operate in the linear region in the 0-V
to 5-V range needed by the ADC. The FFT of a 10-VPP sine wave using a 5.2-V rail and a –0.2-V rail is illustrated
in 图 80. In this test the SNR was calculated to be 102.535 dB and the THD was calculated to be –121.66 dB.
Using –0.2-V and 5.2-V rail voltages still allows for common 5-V (5.5 V max) op amps to be used in the design.
8.2.2.3 Detailed Design Description
8.2.2.3.1 Regulation of –0.2 V
The TPS7A39 has an innovative feature of regulating the negative rail down to zero volts. This regulation is
achieved by using an inverting amplifier and using the positive-buffered reference as the input signal to the
amplifier. Regulating to –0.2 V eliminates the nonlinearity and distortion present when using the full rail range of
the amplifiers.
8.2.2.3.2 Feedback Resistor Selection
Use 公式 17 and 公式 18 to calculate the values of the feedback resistors:
VOUTP = VFBP × (1 + R1P / R2P
)
(17)
(18)
VOUTN = VBUF × (–R1N / R2N
)
For this design the recommended 10-kΩ resistors are used for R2P and R2N. R1P and R1N can be calculated by
substituting R2P and R2N into 公式 19 and 公式 20 because R2P and R2N are already selected.
R1P = [(VOUTP / VFBP) – 1] × R2P = [(5.2 V / 1.188 V) – 1] × 10 kΩ = 33.8 kΩ
R1N = –VOUTN × R2N / VBUF = –(–5 V) × 10 kΩ / 1.19 V = 1.68 kΩ
(19)
(20)
After solving for 公式 19 and 公式 20, the closest one percent resistors are selected, R1N = 1.69 kΩ and R1P = 34
kΩ.
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