Datasheet AD7895 (Analog Devices) - 7

AD7895 CONVERTER DETAILS
ance stage of the track/hold amplifier. For the AD7895-10, The AD7895 is a fast, 12-bit single supply A/D converter. It R1 = 30 kΩ, R2 = 7.5 kΩ and R3 = 10 kΩ. For the AD7895-3, provides the user with signal scaling, track/hold, A/D converter R1 = R2 = 6.5 kΩ and R3 is open circuit. and serial interface logic functions on a single chip. The A/D For the AD7895-10 and AD7895-3, the designed code transi- converter section of the AD7895 consists of a conventional tions occur on successive integer LSB values (i.e., 1 LSB, 2 LSBs, successive-approximation converter based around an R-2R 3 LSBs . .). Output coding is 2s complement binary with 1 LSB ladder structure. The signal scaling on the AD7895-10 and = FS/4096. The ideal input/output transfer function for the AD7895-3 allows the part to handle ± 10 V and ± 2.5 V input AD7895-10 and AD7895-3 is shown in Table I. signals, respectively, while operating from a single +5 V supply. The AD7895-2 accepts an analog input range of 0 V to +2.5 V.
Table I. Ideal Input/Output Code Table for the AD7895-10/-3
The part requires an external +2.5 V reference. The reference
Digital Output
input to the part is buffered on-chip. The AD7895 has two operating modes, the high sampling mode and the auto sleep
Analog Inputl Code Transition
mode, where the part automatically goes into sleep after the end of conversion. These modes are discussed in more detail in the +FSR/2 – 1 LSB2 011 . 110 to 011 . 111 “Timing and Control” section. +FSR/2 – 2 LSBs 011 . 101 to 011 . 110 +FSR/2 – 3 LSBs 011 . 100 to 011 . 101 A major advantage of the AD7895 is that it provides all of the above functions in an 8-pin package, either 8-pin mini-DIP or GND + 1 LSB 000 . 000 to 000 . 001 SOIC. This offers the user considerable spacing saving advantages GND 111 . 111 to 000 . 000 over alternative solutions. The AD7895 consumes only 20 mW GND – 1 LSB 111 . 110 to 111 . 111 maximum, making it ideal for battery-powered applications. –FSR/2 + 3 LSBs 100 . 010 to 100 . 011 Conversion is initiated on the AD7895 by pulsing the CONVST –FSR/2 + 2 LSBs 100 . 001 to 100 . 010 input. On the falling edge of CONVST, the on-chip track/hold –FSR/2 + 1 LSB 100 . 000 to 100 . 001 goes from track to hold mode, and the conversion sequence is started. The conversion clock for the part is generated internally NOTES using a laser-trimmed clock oscillator circuit. Conversion time 1FSR is full-scale range = 20 V (AD7895-10) and = 5 V (AD7895-3) for the AD7895 is 3.8 µs in the high sampling mode (9.8 µs for with REF IN = +2.5 V. 21 LSB = FSR/4096 = 4.883 mV (AD7895-10) and 1.22 mV (AD7895-3) the auto sleep mode), and the track/hold acquisition time is with REF IN = +2.5 V. 0.3 µs. To obtain optimum performance from the part, the read operation should not occur during the conversion or during The analog input section for the AD7895-2 contains no biasing resistors, and the V 300 ns prior to the next conversion. This allows the part to IN pin drives the input to the track/hold amplifier directly. The analog input range is 0 V to +2.5 V into operate at throughput rates up to 192 kHz and achieve data sheet specifications. a high impedance stage with an input current of less than 500 nA. This input is benign with no dynamic charging cur- rents. Once again, the designed code transitions occur on succes-
CIRCUIT DESCRIPTION
sive integer LSB values. Output coding is straight (natural) binary
Analog Input Section
with 1 LSB = FS/4096 = 2.5 V/4096 = 0.61 mV. Table II shows The AD7895 is offered as three part types: the AD7895-10, the ideal input/output transfer function for the AD7895-2. which handles a ± 10 V input voltage range; the AD7895-3, which handles input voltage range ± 2.5 V; and the AD7895-2, which handles a 0 V to +2.5 V input voltage range.
Table II. Ideal Input/Output Code Table for AD7895-2 Digital Output Analog Input1 Code Transition REF IN
+FSR – 1 LSB2 111 . 110 to 111 . 111
TO ADC
+FSR – 2 LSB 111 . 101 to 111 . 110
REFERENCE CIRCUITRY
+FSR – 3 LSB 111 . 100 to 111 . 101
R2 R1 V TO INTERNAL
GND + 3 LSB 000 . 010 to 000 . 011
IN COMPARATOR TRACK/
GND + 2 LSB 000 . 001 to 000 . 010
R3 HOLD
GND + 1 LSB 000 . 000 to 000 . 001
AGND
NOTES
AD7895-10/AD7895-3
1FSR is full-scale range and is 2.5 V for AD7895-2 with VREF = +2.5 V. 21 LSB = FSR/4096 and is 0.61 mV for AD7895-2 with VREF = +2.5 V.
Track/Hold Section
Figure 2. AD7895-10/AD7895-3 Analog Input Structure The track/hold amplifier on the analog input of the AD7895 Figure 2 shows the analog input section for the AD7895-10 and allows the ADC to accurately convert an input sine wave of full- AD7895-3. The analog input range of the AD7895-10 is ± 10 V scale amplitude to 12-bit accuracy. The input bandwidth of the into an input resistance of typically 33 kΩ. The analog input track/hold is greater than the Nyquist rate of the ADC even range of the AD7895-3 is ± 2.5 V into an input resistance of when the ADC is operated at its maximum throughput rate of typically 12 kΩ. This input is benign with no dynamic charging 192 kHz (i.e., the track/hold can handle input frequencies in currents, as the resistor stage is followed by a high input imped- excess of 100 kHz). –6– REV. 0