Datasheet AD4000, AD4004, AD4008 (Analog Devices) - 20

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AD4000/ AD4004/ AD4008 Data Sheet ANALOG INPUTS
The external RC filter is usually present at the ADC input Figure 35 shows an equivalent circuit of the analog input structure, to band limit the input signal. During an overvoltage event, including the overvoltage clamp of the AD4000/AD4004/AD4008. excessive voltage is dropped across REXT, and REXT becomes part of a protection circuit. The REXT value can vary from 200 Ω to
REF
20 kΩ for 15 V protection. The CEXT value can be as low as 100 pF
D1
for correct operation of the clamp. See Table 1 for input overvoltage
C R IN 0V TO 15V EXT IN+ RIN
clamp specifications. The analog input structure allows the sampling of the true
V C C D2 IN EXT PIN CLAMP
differential signal between IN+ and IN−. By using these 010 differential inputs, signals common to both inputs are rejected.
GND
14956- By using IN− to sense a remote signal ground, ground potential Figure 35. Equivalent Analog Input Circuit differences between the sensor and the local ADC ground are
Input Overvoltage Clamp Circuit
eliminated. Most ADC analog inputs, IN+ and IN−, have no overvoltage
Switched Capacitor Input
protection circuitry apart from ESD protection diodes. During During the acquisition phase, the impedance of the analog an overvoltage event, an ESD protection diode from an analog inputs (IN+ or IN−) can be modeled as a parallel combination input pin (IN+ or IN−) to REF forward biases and shorts the of Capacitor CPIN and the network formed by the series connection input pin to REF, potentially overloading the reference or of RIN and CIN. CPIN is primarily the pin capacitance. RIN is typical y causing damage to the device. The AD4000/AD4004/AD4008 400 Ω and is a lumped component composed of serial resistors internal overvoltage clamp circuit with a larger external resistor and the on resistance of the switches. CIN is typically 40 pF and (REXT = 200 Ω) eliminates the need for external protection is mainly the ADC sampling capacitor. diodes and protects the ADC inputs against dc overvoltages. During the conversion phase, where the switches are open, the In applications where the amplifier rails are greater than VREF input impedance is limited to CPIN. RIN and CIN make a single- and less than ground, it is possible for the output to exceed pole, low-pass filter that reduces undesirable aliasing effects and the input voltage range of the device. In this case, the limits noise. AD4000/AD4004/AD4008 internal voltage clamp circuit
RC Filter Values
ensures that the voltage on the input pin does not exceed VREF + The RC filter value (represented by R and C in Figure 33 and 0.4 V and prevents damage to the device by clamping the input voltage in a safe operating range and avoiding disturbance of Figure 34) and driving amplifier can be selected depending on the reference, which is particularly important for systems that the input signal bandwidth of interest at the full throughput. share the reference among multiple ADCs. Lower input signal bandwidth means that the RC cutoff can be lower, thereby reducing noise into the converter. For optimum If the analog input exceeds the reference voltage by 0.4 V, the performance at various throughputs, use the recommended RC internal clamp circuit turns on and the current flows through values (200 Ω, 180 pF) and the ADA4805-1. the clamp into ground, preventing the input from rising further and potentially causing damage to the device. The clamp turns The RC values shown in Table 10 are chosen for ease of drive on before D1 (see Figure 35) and can sink up to 50 mA of current. considerations and greater ADC input protection. The combination of a large R value (200 Ω) and small C value When the clamp is active, it sets the overvoltage (OV) clamp flag results in a reduced dynamic load for the amplifier to drive. bit in the register that can be read back (see Table 14), which is The smal er value of C means fewer stability and phase margin a sticky bit that must be read to be cleared. The status of the concerns with the amplifier. The large value of R limits the current clamp can also be checked in the status bits using an OV clamp into the ADC input when the amplifier output exceeds the ADC flag (see Table 15). The clamp circuit does not dissipate static input range. power in the off state. Note that the clamp cannot sustain the overvoltage condition for an indefinite amount of time.
Table 10. RC Filter and Amplifier Selection for Various Input Bandwidths Input Signal Bandwidth (kHz) R (Ω) C (pF) Recommended Amplifier
<10 See the High-Z Mode section See the High-Z Mode section See the High-Z Mode section <200 200 180 ADA4805-1 >200 200 120 ADA4897-1 Multiplexed 200 120 ADA4897-1 Rev. C | Page 20 of 36 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS TERMINOLOGY THEORY OF OPERATION CIRCUIT INFORMATION CONVERTER OPERATION TRANSFER FUNCTIONS APPLICATIONS INFORMATION TYPICAL APPLICATION DIAGRAMS ANALOG INPUTS Input Overvoltage Clamp Circuit Switched Capacitor Input RC Filter Values DRIVER AMPLIFIER CHOICE High Frequency Input Signals Multiplexed Applications EASE OF DRIVE FEATURES Input Span Compression High-Z Mode Long Acquisition Phase VOLTAGE REFERENCE INPUT POWER SUPPLY DIGITAL INTERFACE REGISTER READ/WRITE FUNCTIONALITY STATUS WORD CSB MODE, 3-WIRE TURBO MODE CSB MODE, 3-WIRE WITHOUT BUSY INDICATOR CSB MODE, 3-WIRE WITH BUSY INDICATOR CSB MODE, 4-WIRE TURBO MODE CSB MODE, 4-WIRE WITHOUT BUSY INDICATOR CSB MODE, 4-WIRE WITH BUSY INDICATOR DAISY-CHAIN MODE LAYOUT GUIDELINES EVALUATING THE AD4000/AD4004/AD4008 PERFORMANCE OUTLINE DIMENSIONS ORDERING GUIDE