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ADCS7476, ADCS7477, ADCS7478

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SNAS192G – APRIL 2003 – REVISED MAY 2016

Product Folder Links: ADCS7476 ADCS7477 ADCS7478

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Copyright © 2003–2016, Texas Instruments Incorporated

Typical Application (continued)

8.2.3 Application Curves

Figure 33. ADCS7476/77 Ideal Transfer Characteristic

Figure 34. ADCS7478 Ideal Transfer Characteristic

9 Power Supply Recommendations

There are three concerns relating to the power supply of these products: the effects of Power Supply Noise upon

the conversion process, the Digital Output Effect Upon Noise upon the conversion process, and Power

Management of the product.

9.1

Power Supply Noise

Because the supply voltage of the ADCS747x is the reference voltage, any noise greater than 1/2 LSB in

amplitude has some effect upon the converter noise performance. This effect is proportional to the input voltage

level. The power supply must receive all the considerations of a reference voltage as far as stability and noise is

concerned. Using the same supply voltage for these devices as is used for digital components leads to degraded

noise performance.

9.2

Digital Output Effect Upon Noise

required from the supply to charge the output capacitance causes voltage variations at the ADC supply line. If

these variations are large enough, they could degrade SNR and SINAD performance of the ADC. Similarly,

discharging the output capacitance when the digital output goes from a logic high to a logic low dumps current

into the die substrate, causing ground bounce noise in the substrate that degrades noise performance if that

current is large enough. The larger the output capacitance, the more current flows through the device power

supply line and die substrate and the greater is the noise coupled into the analog path.

The first solution to keeping digital noise out of the power supply is to decouple the supply from any other

components or use a separate supply for the ADC. To keep noise out of the supply, keep the output load

capacitance as small as practical. If the load capacitance is greater than 50 pF, use a 100-

Ω series resistor at

the ADC output, located as close to the ADC output pin as practical. This limits the charge and discharge current

of the output capacitance and improve noise performance. Because the series resistor and the load capacitance

form a low frequency pole, verify signal integrity when the series resistor is added.

9.3 Power Management

When ADCS747x is operated continuously in normal mode, throughput up to 1 MSPS can be achieved. The user

may trade throughput for power consumption by simply performing fewer conversions per unit time and putting

the ADCS747x into shutdown mode between conversions. This method is not advantageous beyond 350-kSPS

throughput.