Datasheet

23
ADCS7476 , ADCS7477 , ADCS7478
www.ti.com SNAS192G –APRIL 2003–REVISED MAY 2016
Product Folder Links: ADCS7476 ADCS7477 ADCS7478
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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
The charging of any output load capacitance requires current from the digital supply, V
DD
. The current pulses
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.