Given:
Absolute Scale System—PGA309 connected to a system ADC (see Figure 2-25)
System ADC Reference: VREF ADC = 4.096V
PGA309 Reference: VREF = 4.096V (use PGA309 internal reference)
Operating Temperature Range: −40°C to +125°C
PGA309 VSA, VSD = +5V
External Fault Monitor; Trip High when Fault Detected
Find:
Recommended levels to allow for Over/Under-Scale Limits as well as Fault Detection.
- Over-Scale Limit
- Under-Scale Limit
- Useable Linear PGA309 Output Range
- System ADC Trip Points: Over-Scale, Under-Scale, Fault Detect
Solution:
- Analyze the worst case offset errors on the over-scale and under-scale comparators over the operating temperature range. Table 2-20 contains key electrical characteristics needed for this computation.
Over-Scale Comparator Offset Calculation:
Over-Scale Temperature Drift:
−40°C to 25°C: −24.05mV = (+0.37mV/°C)(−40°C − 25°C)
25°C to +125°C: +37.00mV = (+0.37mV/°C)(+125°C − 25°C)
Over-Scale Offset Min and Max:
VOS min = +6mV −24.05mV = −18.05mV
VOS max = +114mV + 37.00mV = +151.00mV
Under-Scale Comparator Offset Calculation:
Under-Scale Temperature Drift:
−40°C to 25°C: +9.75mV = (−0.15mV/°C)(−40°C − 25°C)
25°C to +125°C: −15.00mV = (−0.15mV/°C)(+125°C − 25°C)
Under-Scale Offset Min and Max:
VUS min = −7mV + 9.75mV = −2.75mV
VUS max = −93mV −15.00mV = −108mV - Analyze the worst-case change in VREF over the operating temperature range.
VREF Temperature Drift:
−40°C to +125°C: [(+10ppm/°C)/(1e6)][+125°C − (−40°C)]VREF = +0.00165 VREF
VREF Min and Max:
VREF min = 4.00V – (0.00165)(4.00V) = 3.9934V
VREF max = 4.14V + (0.00165)(4.00V) = 4.1466V - Calculate the over-scale and under-scale min and max trip points over the operating temperature range for each overscale and under-scale threshold (refer to Table 2-21).
Over-Scale (OS) Min and Max Trip Points:
OS min = VREF min (OS ratio) + VOS min
OS max = VREF max (OS ratio) + VOS max
Under-Scale (US) Min and Max Trip Points:
US min = VREF min (US ratio) + VUS max
US max = VREF max (US ratio) + VUS min - From the over-scale and under-scale min and max trip point calculations, choose the best selection that will allow for the optimum system ADC range budget (see Figure 2-26). For this example, the PGA309 is scalable for a linear output of 8% to 80.8% of the system ADC reference. In addition, we can set reasonable trip points for detecting over-scale limit, under-scale limit, and fault detect.
- Check that the PGA309 VOUT can support the voltage swings defined in the System ADC range budget. Table 2-22 confirms that for our example the PGA309 VOUT can meet the limiting conditions for our desired scaling.
Since the PGA309 + sensor is usually calibrated together as a system, the over-scale and under-scale limits can be measured per device at the operating temperature extremes, and the final limits adjusted as desired for optimum scaling. In a ratiometrically scaled system, the reference error will not need to be included in the over-scale and under-scale trip point calculations.