AD7874 Gain error can be adjusted at either the first code transition (ADC negative full scale) or the last code transition (ADC positive full scale). The trim procedures for both cases are as follows:
Positive Full-Scale Adjust
Apply a voltage of +9.9927 V (FS/2 – 3/2 LSBs) at V1. Adjust R2 until the ADC output code flickers between 0111 1111 1110 and 0111 1111 1111.
000...010 000...001 000...000
– FS 2 + FS – 1LSB 2
FS=20V 1LSB = FS 4096
0V INPUT VOLTAGE
Figure 5. Input/Output Transfer Function OFFSET AND FULL-SCALE ADJUSTMENT
In most Digital Signal Processing (DSP) applications, offset and full-scale errors have little or no effect on system performance. Offset error can always be eliminated in the analog domain by ac coupling. Full-scale error effect is linear and does not cause problems as long as the input signal is within the full dynamic range of the ADC. Invariably, some applications will require that the input signal span the full analog input dynamic range. In such applications, offset and full-scale error will have to be adjusted to zero. Figure 6 shows a circuit which can be used to adjust the offset and full-scale errors on the AD7874 (Channel 1 is shown for example purposes only). Where adjustment is required, offset error must be adjusted before full-scale error. This is achieved by trimming the offset of the op amp driving the analog input of the AD7874 while the input voltage is a 1/2 LSB below analog ground. The trim procedure is as follows: apply a voltage of –2.44 mV (–1/2 LSB) at V1 in Figure 6 and adjust the op amp offset voltage until the ADC output code flickers between 1111 1111 1111 and 0000 0000 0000. INPUT RANGE = ±10V V 1 R1 10kΩ R2 500Ω
VIN1 R4 10kΩ
R5 10kΩ AGND
- ADDITIONAL PINS OMITTED FOR CLARITY
Figure 6. AD7874 Full-Scale Adjust Circuit
Negative Full-Scale Adjust
Apply a voltage of –9.9976 V ( –FS + 1/2 LSB) at V1 and adjust R2 until the ADC output code flickers between 1000 0000 0000 and 1000 0000 0001. An alternative scheme for adjusting full-scale error in systems which use an external reference is to adjust the voltage at the REF IN pin until the full-scale error for any of the channels is adjusted out. The good full-scale matching of the channels will ensure small full-scale errors on the other channels. TIMING AND CONTROL
Conversion is initiated on the AD7874 by asserting the CONVST input. This CONVST input is an asynchronous input which is independent of the ADC clock. This is essential for applications where precise sampling in time is important. In these applications, the signal sampling must occur at exactly equal intervals to minimize errors due to sampling uncertainty or jitter. In these cases, the CONVST input is driven from a timer or precise clock source. Once conversion is started, CONVST should not be asserted again until conversion is complete on all four channels. In applications where precise time interval sampling is not critical, the CONVST pulse can be generated from a microprocessor WRITE or READ line gated with a decoded address (different to the AD7874 CS address). CONVST should not be derived from a decoded address alone because very short CONVST pulses (which may occur in some microprocessor systems as the address bus is changing at the start of an instruction cycle) could initiate a conversion. All four track/hold amplifiers go from track to hold on the rising edge of the CONVST pulse. The four track/hold amplifiers remain in their hold mode while all four channels are converted. The rising edge of CONVST also initiates a conversion on the Channel 1 input voltage (VIN1). When conversion is complete on Channel 1, its result is stored in Data Register 1, one of four on-chip registers used to store the conversion results. When the result from the first conversion is stored, conversion is initiated on the voltage held by track/hold 2. When conversion has been completed on the voltage held by track/hold 4 and its result is stored in Data Register 4, INT goes low to indicate that the conversion process is complete. The sequence in which the channel conversions takes place is automatically taken care of by the AD7874. This means that the user does not have to provide address lines to the AD7874 or worry about selecting which channel is to be digitized. Reading data from the device consists of four read operations to the same microprocessor address. Addressing of the four on-chip data registers is again automatically taken care of by the AD7874.