Datasheet AD7938-6 (Analog Devices) - 21

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AD7938-6 Data Sheet 220Ω
is −0.1 V to +0.4 V; however, typically this range can extend to
2 × VREF p-p
−0.3 V to +0.7 V when V
440Ω V+
DD = 3 V, or −0.3 V to +1.8 V when
GND 27Ω 3.75V
VDD = 5 V. Figure 30 shows a connection diagram for pseudo
2.5V 1.25V
differential mode.
V– VIN+ 220Ω 220Ω AD7938-6 VREF p-p 220Ω VIN+ V+ VIN– VREF 3.75V AD7938-6* 2.5V 27Ω V A 1.25V IN– V– VREF 0.47µF DC INPUT 10kΩ
5
VOLTAGE 20kΩ
03
0.47µF
1- 75 7 04 03 1- Figure 28. Dual Op Amp Circuit to Convert a Single-Ended 75
*ADDITIONAL PINS OMITTED FOR CLARITY.
04 Bipolar Signal into a Differential Unipolar Signal Figure 30. Pseudo Differential Mode Connection Diagram
220Ω VREF p-p ANALOG INPUT SELECTION VREF 440Ω V+ 27Ω 3.75V
As shown in Table 9, the user can set up their analog input
GND 2.5V 1.25V
configuration by setting the values in the MODE0 and MODE1
V– VIN+
bits in the control register. Assuming the configuration has been
220Ω AD7938-6
chosen, there are different ways of selecting the analog input to
220Ω
be converted depending on the state of the SEQ and SHDW bits
V+ VIN– VREF 3.75V
in the control register.
2.5V 27Ω A 1.25V Traditional Multichannel Operation (SEQ = 0, SHDW = 0) V–
Any one of eight analog input channels or four pairs of channels
0.47µF 10kΩ 20kΩ
36 can be selected for conversion in any order by setting the SEQ 0 1- 75 and SHDW bits in the control register to 0. The channel to be 04 Figure 29. Dual Op Amp Circuit to Convert a Single-Ended converted is selected by writing to the address bits, ADD2 to Unipolar Signal into a Differential Signal ADD0, in the control register to program the multiplexer prior to the conversion. This mode of operation is that of a traditional Another method of driving the AD7938-6 is to use the AD8138 multichannel ADC where each data write selects the next (or equivalent) differential amplifier. The AD8138 can be used channel for conversion. Figure 31 shows a flow chart of this as a single-ended-to-differential amplifier or as a differential-to- mode of operation. The channel configurations are shown in differential amplifier. The device is as easy to use as an op amp Table 9. and greatly simplifies differential signal amplification and driving.
Pseudo Differential Mode POWER ON
The AD7938-6 can have four pseudo differential pairs (Pseudo
WRITE TO THE CONTROL REGISTER TO SET UP OPERATING MODE, ANALOG INPUT
Mode 1) or seven pseudo differential inputs (Pseudo Mode 2)
AND OUTPUT CONFIGURATION SET SEQ = SHDW = 0. SELECT THE DESIRED
by setting the MODE0 and MODE1 bits in the control register
CHANNEL TO CONVERT (ADD2 TO ADD0).
to 1, 0 and 1, 1, respectively. In the case of the four pseudo differential pairs, VIN+ is connected to the signal source, which
ISSUE CONVST PULSE TO INITIATE A CONVERSION ON THE SELECTED CHANNEL.
must have an amplitude of VREF (or 2 × VREF depending on the
INITIATE A READ CYCLE TO READ THE DATA FROM THE SELECTED CHANNEL.
range chosen) to make use of the full dynamic range of the part. A dc input is applied to the VIN− pin. The voltage applied to this
INITIATE A WRITE CYCLE TO SELECT THE NEXT
input provides an offset from ground or a pseudo ground for
CHANNEL TO BE CONVERTED BY
38
CHANGING THE VALUES OF BITS ADD2 TO ADD0
0 1- the V
IN THE CONTROL REGISTER. SEQ = SHDW = 0.
75 IN+ input. In the case of the seven pseudo differential 04 inputs, the seven analog input signals inputs are referred to a dc Figure 31. Traditional Multichannel Operation Flow Chart voltage applied to VIN7. The benefit of pseudo differential inputs is that they separate the analog input signal ground from the ADC ground allowing dc common-mode voltages to be cancelled. The specified voltage range for the VIN− pin while in pseudo differential mode Rev. D | Page 20 of 32 Document Outline FEATURES FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION PRODUCT HIGHLIGHTS REVISION HISTORY SPECIFICATIONS TIMING SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS TERMINOLOGY ON-CHIP REGISTERS CONTROL REGISTER SEQUENCER OPERATION Writing to the Control Register to Program the Sequencer SHADOW REGISTER CIRCUIT INFORMATION CONVERTER OPERATION ADC TRANSFER FUNCTION TYPICAL CONNECTION DIAGRAM ANALOG INPUT STRUCTURE ANALOG INPUTS Single-Ended Mode Differential Mode Driving Differential Inputs Using an Op Amp Pair Pseudo Differential Mode ANALOG INPUT SELECTION Traditional Multichannel Operation (SEQ = 0, SHDW = 0) Using the Sequencer: Programmable Sequence (SEQ = 0, SHDW = 1 ) Consecutive Sequence (SEQ = 1, SHDW = 1) REFERENCE Digital Inputs VDRIVE Input PARALLEL INTERFACE Reading Data from the AD7938-6 Writing Data to the AD7938-6 POWER MODES OF OPERATION Normal Mode (PM1 = PM0 = 0) Autoshutdown (PM1 = 0; PM0 = 1) Autostandby (PM1 = 1; PM0 = 0) Full Shutdown Mode (PM1 =1; PM0 = 1) POWER vs. THROUGHPUT RATE MICROPROCESSOR INTERFACING AD7938-6 to ADSP-21xx Interface AD7938-6 to ADSP-21065L Interface AD7938-6 to TMS32020, TMS320C25, and TMS320C5x Interface AD7938-6 to 80C186 Interface APPLICATION HINTS GROUNDING AND LAYOUT PCB DESIGN GUIDELINES FOR CHIP SCALE PACKAGE EVALUATING THE AD7938-6 PERFORMANCE OUTLINE DIMENSIONS ORDERING GUIDE