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ADS7833N Arkusz danych(PDF) 9 Page - Burr-Brown (TI) |
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ADS7833N Arkusz danych(HTML) 9 Page - Burr-Brown (TI) |
9 / 12 page ® ADS7833 9 PGA GAIN The PGA gain is determined by the Gain Select portion (bits 8 and 9) in the SERIN word (see Figure 2). There is one gain input that sets the same gain for all three PGAs. The gain values and allowable full scale inputs are shown in Table II. For channels one and two the PGAs set the gain for all three analog inputs. For the third channel, only the V 3-1 input is gain changed by the PGA. Inputs V3-2, V3-3 and V3-4 are connected to ADC 3 at a fixed gain of 1V/V regardless of the Gain Select value. Input Select = 3H—Convert V1-3 via SH2, V2-3 via SH4, and V 3-3 (V1-3 and V2-3 are from the value sampled in a preceding conversion cycle with Input Select = 4H, 5H or 6 H). Input Select = 2H—Convert V1-3 via SH1, V2-3 via SH3, and V3-3 (V1-3 is sampled on SH2 in this conversion cycle). Input Select = 1H—Input V3-4 is converted by PGA3/ ADC3. The output of the asynchronous sample holds, SH6 and SH 7, are converted by PGA1/ADC1 and PGA2/ADC2, respectively. Note that the inputs to SH6 and SH7 are determined by previous Input Select values (see Table IV). Thus, to properly convert the output of one of the asynchro- nous sample holds it is first necessary to choose its input with a previous conversion cycle. Also, the output of SH6 or SH 7 will only be converted if ASH goes low before the CONV command is received. Input Select = 0H—V3-4 is converted by PGA3/ADC3. The inputs to PGA1/ADC1 and PGA2/ADC2 are undefined. CONVERSIONS FROM THE ASYNCHRONOUS SAMPLE HOLDS Decoding the Input Select value also determines which inputs are applied to the two asynchronously controlled sample holds SH6 and SH7. (See Table IV.) One of the three possible inputs is selected by the Input Select value being 4, 5, or 6. The “No Effect” states indicate that these values of Input Select have no effect on the multiplexers at the input of SH6 and SH7. When one of the “No Effect” values of Input Select is presented, the multiplexers will not be changed (i.e., their condition is determined by the last 4, 5, or 6 value of Input Select that existed prior to the “No Effect” state). Note that Input Select = 1 H presents the output of SH6 and SH7 (1ASHX and 2ASHX) to PGA1/ADCl and PGA2/ADC2, respectively (see Table III). Therefore, in order to properly convert the asynchronous sampled signals, it is first neces- sary to choose an input signal (Input Select equal 5 or 6 in Table IV) with one load/convert cycle and then convert the sample hold output (Input Select = 4 in Table III) in a following conversion cycle. GAIN GAIN FULL SCALE SELECT0-1 SETTING INPUT 0H 5.0V/V ±0.5V 1H 2.5V/V ±1.0V 2H 1.25V/V ±2.0V 3H 1.0V/V ±2.5V TABLE II. Gain Select Information. INPUT MULTIPLEXER AND SAMPLE HOLD SELECTION The Input Select portion of the SERIN word (bits 10, 11 and 12) (see Figure 2) are decoded and determine the open/closed condition of the multiplexer switches. This in turn determines which input signals are connected to the sample holds and which sample holds are connected to the PGAs/ADCs. INPUT SIGNALS FOR PGAs/ADCs Table III shows the relationships between the value of Input Select0-2 and the signals that are converted. INPUT SELECT0-2 ANALOG SIGNAL CONNECTED TO PGAX/ADCX PGA1/ADC1 PGA2/ADC2 PGA3/ADC2 0H 000 Undefined Undefined V3-4 1H 001 V1-X via SH6(1) V2-X via SH7(1) V3-4 2H 010 V1-3 via SH1 V2-3 via SH3 V3-3 3H 011 V1-3 via SH2 V2-3 via SH4 V3-3 4H 100 V1-2 V2-2 V3-2 5H 101 V1-2 V2-2 V3-2 6H 110 V1-2 V2-2 V3-2 7H 111 V1-1 V2-1 V3-1 NOTE: (1) See Table IV for Operation. HEX BINARY CODE CODE TABLE III. Input Controls for Synchronous Sample Holds. Input Select = 7H—Synchronously sample and convert input signals V1-1, V2-1, and V3-1. Input Select = 4H, 5H, 6H—Synchronously sample and convert input signals V 1-2, V2-2, and V3-2. These codes also cause SH2 and SH4 to sample their inputs. Values 4H, 5H, 6H have different effects on the inputs to SH 6 and SH7 (see Table IV). INPUT SELECT0-2 ANALOG SIGNAL CONNECTED TO SH6 SH7 0H 000 No Effect No Effect 1H 001 No Effect No Effect 2H 010 No Effect No Effect 3H 011 No Effect No Effect 4H 100 Open Open 5H 101 V1-3 V2-3 6H 110 V1-2 V1-2 7H 111 No Effect No Effect HEX BINARY CODE CODE TABLE IV. Input Controls for Asynchronous Sample Holds. |
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