AD767 THE AD767 OFFERS TRUE 12-BIT PERFORMANCE OVER THE FULL TEMPERATURE RANGE
LINEARITY ERROR: Analog Devices defines linearity error as the maximum deviation of the actual, adjusted DAC output from the ideal analog output (a straight line drawn from 0 to F.S. – 1 LSB) for any bit combination. This is also referred to as relative accuracy. The AD767 is laser trimmed to typically maintain linearity errors at less than ± 1/8 LSB for the K and B versions and ± 1/2 LSB for the J, A and S versions. Linearity over temperature is also held to ± 1/2 LSB (K/B) or ± 1 LSB (J/A/S).
Gain and offset drift are minimized in the AD767 because of the thermal tracking of the scaling resistors with other device components. Connections for various output voltage ranges are shown in Table I.
MONOTONICITY: A DAC is said to be monotonic if the output either increases or remains constant for increasing digital inputs such that the output will always be a nondecreasing function of input. All versions of the AD767 are monotonic over their full operating temperature range. DIFFERENTIAL NONLINEARITY: Monotonic behavior requires that the differential linearity error be less than 1 LSB both at +25°C as well as over the temperature range of interest. Differential nonlinearity is the measure of the variation in analog value, normalized to full scale, associated with a 1 LSB change in digital input code. For example, for a 10 volt full-scale output, a change of 1 LSB in digital input code should result in a 2.44 mV change in the analog output (1 LSB = 10 V ϫ 1/4096 = 2.44 mV). If in actual use, however, a 1 LSB change in the input code results in a change of only 0.61 mV (1/4 LSB) in analog output, the differential nonlinearity error would be –1.83 mV, or –3/4 LSB.
Figure 1. Output Amplifier Voltage Range Scaling Circuit UNIPOLAR CONFIGURATION (Figure 2)
This configuration will provide a unipolar 0 to +10 volt output range. In this mode, the bipolar offset terminal, Pin 4, should be grounded if not used for trimming. STEP I … ZERO ADJUST Turn all bits OFF and adjust zero trimmer R1, until the output reads 0.000 volts (1 LSB = 2.44 mV). In most cases this trim is not needed, and Pin 4 should be connected to Pin 5. STEP II … GAIN ADJUST Turn all bits ON and adjust 100 Ω gain trimmer R2 until the output is 9.9976 volts. (Full scale is adjusted to 1 LSB less than nominal full scale of 10.000 volts.)
GAIN ERROR: DAC gain error is a measure of the difference between an ideal DAC and the actual device’s output span. All grades of the AD767 have a maximum gain error of 0.2% FS. However, if this is not sufficient, the error can easily be adjusted to zero (see Figures 2 and 3). UNIPOLAR OFFSET ERROR: Unipolar offset error is a combination of the offset errors of the voltage-mode DAC and the output amplifier and is measured when the AD767 is configured for unipolar outputs. It is present for all codes and is measured with all “0s” in the DAC latches. This is easily adjustable to zero when required. BIPOLAR ZERO ERROR: Bipolar zero errors result from errors produced by the DAC and output amplifier when the AD767 is configured for bipolar output. Again, as with unipolar offset and gain errors, this is easily adjusted to zero when required.
Figure 2. 0 to +10 V Unipolar Voltage Output
ANALOG CIRCUIT CONNECTIONS
Internal scaling resistors provided in the AD767 may be connected to produce bipolar output voltage ranges of ± 10, ± 5 or ±2.5 V or unipolar output voltage ranges of 0 to +5 V or 0 to +10 V. Table I. Output Voltage Range Connections
Digital Input Codes
Connect Pin 9 to
Connect Pin 1 to
Connect Pin 2 to
Connect Pin 4 to
± 10 V ±5 V ± 2.5 V 0 to +10 V 0 to +5 V
Offset Binary Offset Binary Offset Binary Straight Binary Straight Binary
1 1 and 2 2 1 and 2 2
9 2 and 9 3 2 and 9 3
NC 1 and 9 9 1 and 9 9
6 (through 50 Ω fixed or 100 Ω trim resistor) 6 (through 50 Ω fixed or 100 Ω trim resistor) 6 (through 50 Ω fixed or 100 Ω trim resistor) 5 (or optional trim – See Figure 2) 5 (or optional trim – See Figure 2)