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AD7741BN датащи(PDF) 7 Page - Analog Devices |
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AD7741BN датащи(HTML) 7 Page - Analog Devices |
7 / 12 page REV. 0 AD7741/AD7742 –7– TERMINOLOGY INTEGRAL NONLINEARITY For the VFC, Integral Nonlinearity (INL) is a measure of the maximum deviation from a straight line passing through the actual endpoints of the VFC transfer function. The error is expressed in % of the frequency span: Frequency Span = fOUT(max) – fOUT(min) OFFSET ERROR This is a measure of the offset error of the VFC. Ideally, the minimum output frequency (corresponding to minimum input voltage) is 5% of fCLKIN The deviation from this value is the offset error. It is expressed in terms of the error referred to the input voltage. It is expressed in mV. GAIN ERROR This is a measure of the span error of the VFC. The gain is the scale factor that relates the input VIN to the output fOUT. The gain error is the deviation in slope of the actual VFC transfer characteristic from the ideal expressed as a percentage of the full-scale span. OFFSET ERROR DRIFT This is a measure of the change in Offset Error with changes in temperature. It is expressed in µV/°C. GAIN ERROR DRIFT This is a measure of the change in Gain Error with changes in temperature. It is expressed in (ppm of span)/ °C. POWER-SUPPLY REJECTION RATIO (PSRR) This indicates how the output of the VFC is affected by changes in the supply voltage. Again, this error is referred to the input voltage. The input voltage is kept constant and the VDD supply is varied ±5%. The ratio of the apparent change in input voltage to the change in VDD is measured in dBs. CHANNEL-TO-CHANNEL ISOLATION This is a ratio of the amplitude of the signal at the input of one channel to a sine wave on the input of another channel. It is measured in dBs. COMMON-MODE REJECTION For the AD7742, the output frequency should remain un- changed provided the differential input remains unchanged although its common-mode level may change. The CMR is the ratio of the apparent change in differential input voltage to the actual change in common-mode voltage. It is expressed in dBs. GENERAL DESCRIPTION The AD7741/AD7742 are a new generation of CMOS synchro- nous Voltage-to-Frequency Converters (VFCs) that use a charge-balance conversion technique. The AD7741 is a single- channel version and the AD7742 is a multichannel version. The input voltage signal is applied to a proprietary programmable gain front-end based around an analog modulator that converts the input voltage into an output pulse train. The parts also contain an on-chip +2.5 V bandgap reference and operate from a single +5 V supply. A block diagram of the AD7742 is shown in Figure 2. INTEGRATOR COMPARATOR SWITCHED CAPS SWITCHED CAPS fOUT INPUT MUX VIN1 VIN2 VIN3 VIN4 Figure 2. AD7742 Block Diagram Input Amplifier Stage The buffered input stage for the analog inputs presents a high impedance, allowing significant external source impedances. The four analog inputs (VIN1 through VIN4) each have a voltage range from +0.5 V to VDD – 1.75 V. This is an absolute voltage range and is relative to the GND pin. In the case of the AD7742 multichannel part, a differential multiplexer switches one of the differential input channels to the VFC modulator. The multiplexer is controlled by two pins, A1 and A0. See Table I for channel configurations. Table I. AD7742 Input Channel Selection A1 A0 VIN(+) VIN(–) Type 00 VIN1VIN4 Pseudo Differential 01 VIN2VIN4 Pseudo Differential 10 VIN3VIN4 Full Differential 11 VIN1VIN2 Full Differential Analog Input Ranges The AD7741 has a unipolar single-ended input channel whereas the AD7742 contains four input channels which may be con- figured as two fully differential channels or as three pseudo- differential channels. The AD7742 also has a X1/X2 gain option on the front end. The channel and gain settings are pin-programmable. The AD7742 uses differential inputs to provide common-mode noise rejection (i.e., the converted result will correspond to the differential voltage between the two inputs). The absolute voltage on both inputs must lie between +0.5 V and VDD –1.75 V. |
Аналогичный номер детали - AD7741BN |
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Аналогичное описание - AD7741BN |
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