ADC124S051_15 Datasheet, PDF (20/28 Pages) Texas Instruments – 12-Bit A/D Converter

ADC124S051
SNAS260E – NOVEMBER 2004 – REVISED NOVEMBER 2004
www.ti.com
ANALOG INPUTS
An equivalent circuit for one of the ADC124S051's input channels is shown in Figure 51. Diodes D1 and D2
provide ESD protection for the analog inputs. At no time should any input go beyond (V_A + 300 mV) or (GND −
300 mV), as these ESD diodes will begin conducting, which could result in erratic operation. For this reason,
these ESD diodes should NOT be used to clamp the input signal.
The capacitor C1 in Figure 51 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
ADC124S051 sampling capacitor, and is typically 30 pF. The ADC124S051 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 using the ADC124S051 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
performance.
V_A
V_IN
C1
3 pF
D1
C2
R1 ^{30 pF}
D2
Conversion Phase - Switch Open
Track Phase - Switch Closed
Figure 51. Equivalent Input Circuit
DIGITAL INPUTS AND OUTPUTS
The ADC124S051's digital output DOUT is limited by, and cannot exceed, the supply voltage, V_A. The digital
input pins are not prone to latch-up and, and although not recommended, SCLK, CS and DIN may be asserted
before V_A without any latch-up risk.
POWER SUPPLY CONSIDERATIONS
The ADC124S051 is fully powered-up whenever CS is low, and fully powered-down whenever CS is high, with
one exception: the ADC124S051 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 ADC124S051 can perform multiple conversions back to back; each conversion requires 16 SCLK cycles.
The ADC124S051 will perform conversions continuously as long as CS is held low.
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 Performance Characteristics section shows the
typical power consumption of the ADC124S051 versus throughput. To calculate the power 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 multiplied by the shutdown mode power dissipation.
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