ADC122S051CIMM/HALF National Semiconductor, ADC122S051CIMM/HALF Datasheet - Page 18

• Manufacturer Part Number: ADC122S051CIMM/HALF
• Manufacturer: National Semiconductor

Specifications of ADC122S051CIMM/HALF

• Lead Free Status / Rohs Status: Supplier Unconfirmed

www.national.com
5.0 ANALOG INPUTS
An equivalent circuit for one of the ADC122S051's input chan-
nels is shown in Figure 5. Diodes D1 and D2 provide ESD
protection for the analog inputs. At no time should any input
go beyond (V
diodes will begin conducting, which could result in erratic op-
eration. For this reason, these ESD diodes should NOT be
used to clamp the input signal.
The capacitor C1 in Figure 5 has a typical value of 3 pF, and
is mainly the package pin capacitance. Resistor R1 is the on
resistance of the multiplexer and track / hold switch, and is
typically 500 ohms. Capacitor C2 is the ADC122S051 sam-
pling capacitor and is typically 30 pF. The ADC122S051 will
deliver best performance when driven by a low-impedance
source to eliminate distortion caused by the charging of the
sampling capacitance. This is especially important when us-
ing the ADC122S051 to sample AC signals. Also important
when sampling dynamic signals is a band-pass or low-pass
filter to reduce harmonics and noise, improving dynamic per-
formance.
6.0 DIGITAL INPUTS AND OUTPUTS
The ADC122S051's digital output DOUT is limited by, and
cannot exceed, the supply voltage, V
are not prone to latch-up and, and although not recommend-
ed, SCLK, CS and DIN may be asserted before V
any latch-up risk.
7.0 POWER SUPPLY CONSIDERATIONS
The ADC122S051 is fully powered-up whenever CS is low,
and fully powered-down whenever CS is high, with one ex-
ception: the ADC122S051 automatically enters power-down
mode between the 16th falling edge of a conversion and the
1st falling edge of the subsequent conversion (see Timing
Diagrams).
The ADC122S051 can perform multiple conversions back to
back; each conversion requires 16 SCLK cycles. The AD-
C122S051 will perform conversions continuously as long as
CS is held low.
FIGURE 5. Equivalent Input Circuit
A
+ 300 mV) or (GND − 300 mV), as these ESD
A
. The digital input pins
20106414
A
without
18
The user may trade off throughput for power consumption by
simply performing fewer conversions per unit time. The Power
Consumption vs. Sample Rate curve in the Typical Perfor-
mance Curves section shows the typical power consumption
of the ADC122S051 versus throughput. To calculate the pow-
er consumption, simply multiply the fraction of time spent in
the normal mode by the normal mode power consumption ,
and add the fraction of time spent in shutdown mode multi-
plied by the shutdown mode power dissipation.
7.1 Power Management
When the ADC122S051 is operated continuously in normal
mode, the maximum throughput is f
be traded for power consumption by running f
imum 8 MHz and performing fewer conversions per unit time,
putting the ADC122S051 into shutdown mode between con-
versions. A plot of typical power consumption versus through-
put is shown in the Typical Performance Curves section. To
calculate the power consumption for a given throughput, mul-
tiply the fraction of time spent in the normal mode by the
normal mode power consumption and add the fraction of time
spent in shutdown mode multiplied by the shutdown mode
power consumption. Generally, the user will put the part into
normal mode and then put the part back into shutdown mode.
Note that the curve of power consumption vs. throughput is
nearly linear. This is because the power consumption in the
shutdown mode is so small that it can be ignored for all prac-
tical purposes.
7.2 Power Supply Noise Considerations
The charging of any output load capacitance requires current
from the power supply, V
the supply to charge the output capacitance will cause voltage
variations on the supply. If these variations are large enough,
they could degrade SNR and SINAD performance of the ADC.
Furthermore, discharging the output capacitance when the
digital output goes from a logic high to a logic low will dump
current into the die substrate, which is resistive. Load dis-
charge currents will cause "ground bounce" noise in the sub-
strate that will degrade noise performance if that current is
large enough. The larger is the output capacitance, the more
current flows through the die substrate and the greater is the
noise coupled into the analog channel, degrading noise per-
formance.
To keep noise out of the power supply, keep the output load
capacitance as small as practical. If the load capacitance is
greater than 35 pF, use a 100 Ω series resistor at the ADC
output, located as close to the ADC output pin as practical.
This will limit the charge and discharge current of the output
capacitance and improve noise performance.
A
. The current pulses required from
SCLK
/16. Throughput may
SCLK
at its max-