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AD7899AR-1 Datasheet(PDF) 8 Page - Analog Devices |
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AD7899AR-1 Datasheet(HTML) 8 Page - Analog Devices |
8 / 16 page REV. A AD7899 –8– CONVERTER DETAILS The AD7899 is a high-speed, low-power, 14-bit A/D converter that operates from a single 5 V supply. The part contains a 2.2 µs successive-approximation ADC, track/hold amplifier, an internal 2.5 V reference and a high-speed parallel interface. The part accepts an analog input range of ±10 V or ±5 V (AD7899-1), 0 V to 2.5 V or 0 V to 5 V (AD7899-2) and ±2.5 V (AD7899-3). Overvoltage protection on the analog inputs for the part allows the input voltage to go to ±18 V (AD7899-1 with ±10 V input range), –9 V to +18 V (AD7899-1 with ±5 V input range), –1 V to +18 V (AD7899-2) and –4 V to +18 V (AD7899-3) without causing damage. A conversion is initiated on the AD7899 by pulsing the CONVST input. On the rising edge of CONVST, the on-chip track/hold is placed into hold and the conversion is started. The BUSY/ EOC output signal is triggered high on the rising edge of CONVST and will remain high for the duration of the conversion sequence. The conversion clock for the part is generated internally using a laser-trimmed clock oscillator circuit. There is also the option of using an external clock. An external noncontinuous clock is applied to the CLKIN pin. If, on the rising edge of CONVST, this input is high, the external clock will be used. The external clock should not start until 100 ns after the rising edge of CONVST. The optimum throughput is obtained by using the internally gener- ated clock—see Using an External Clock. The BUSY/ EOC signal indicates the end of the conversion, and at this time the Track and Hold returns to tracking mode. The conversion results can be read at the end of the conversion (indicated by BUSY/ EOC going low) via a 14-bit parallel data bus with standard CS and RD signals—see Timing and Control. Conversion time for the AD7899 is 2.2 µs and the track/hold acquisition time is 0.3 µs. To obtain optimum performance from the part, the read operation should not occur during a conversion or during the 150 ns prior to the next CONVST rising edge. This allows the part to operate at throughput rates up to 400 kHz and achieve data sheet specifications. CIRCUIT DESCRIPTION Track/Hold Section The track/hold amplifier on the AD7899 allows the ADCs to accurately convert an input sine wave of full-scale amplitude to 14-bit accuracy. The input bandwidth of the track/hold is greater than the Nyquist rate of the ADC even when the ADC is oper- ated at its maximum throughput rate of 400 kSPS (i.e., the track/hold can handle input frequencies in excess of 200 kHz). The track/hold amplifier’s acquire input signals to 14-bit accuracy in less than 300 ns The operation of the track/hold is essentially transparent to the user. The track/hold amplifier samples the input channel on the rising edge of CONVST. The aperture time for the track/hold (i.e., the delay time between the external CONVST signal and the track/hold actually going into hold) is typically 15 ns and, more importantly, is well matched from device to device. It allows multiple AD7899s to sample more than one channel simultaneously. At the end of a conversion, the part returns to its tracking mode. The acquisition time of the track/hold amplifier begins at this point. Reference Section The AD7899 contains a single reference pin, labelled VREF, which either provides access to the part’s own 2.5 V reference or allows an external 2.5 V reference to be connected to provide the reference source for the part. The part is specified with a 2.5 V reference voltage. To use the internal reference as the reference source for the AD7899, simply connect a 0.1 µF capacitor from the VREF pin to AGND. The voltage that appears at this pin is internally buffered before being applied to the ADC. If this reference is required for use external to the AD7899, it should be buffered, as the part has a FET switch in series with the reference output resulting in a source impedance for this output of 6 k Ω nominal. The tolerance on the internal reference is ±10 mV at 25°C with a typical temperature coefficient of 25 ppm/ °C and a maximum error over temperature of ±20 mV. If the application requires a reference with a tighter tolerance or the AD7899 needs to be used with a system reference, the user has the option of connecting an external reference to this VREF pin. The external reference will effectively overdrive the internal reference and thus provide the reference source for the ADC. The reference input is buffered before being applied to the ADC with the maximum input current of ±100 µA. Suitable reference sources for the AD7899 include the AD680, AD780, REF192, and REF43 precision 2.5 V references. Analog Input Section The AD7899 is offered as three part types, the AD7899-1 where the input can be configured for ± 10 V or a ± 5 V input voltage range, the AD7899-2 where the input can be configured for 0 V to 5 V or a 0 V to 2.5 V input voltage range and the AD7899-3 which handles input voltage range ±2.5 V. The amount of current flowing into the analog input will depend on the analog input range and the analog input voltage. The maximum current flows when negative full-scale is applied. AD7899-1 Figure 2 shows the analog input section of the AD7899-1. The input can be configured for ±5 V or ±10 V operation on the AD7899-1. For ±5 V operation, the VINA and VINB inputs are tied together and the input voltage is applied to both. For ±10 V operation, the VINB input is tied to AGND and the input voltage is applied to the VINA input. The VINA and VINB inputs are sym- metrical and fully interchangeable. |
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