PGA280
www.ti.com................................................................................................................................................ SBOS487A –JUNE 2009–REVISED SEPTEMBER 2009
CheckSum Error (CRCerr)
SPI communication can include a checksum byte for increased data integrity, when enabled. It is especially
useful for an isolated SPI. This error detection is only active with the checksum activated. See the Checksum
section for details.
POWER SUPPLY
The PGA280 can connect to three supply voltages: the high-voltage analog supply, the low-voltage output
amplifier supply, and the digital I/O supply. This architecture allows an optimal interface (level-shift) to the
different supply domains.
The high-voltage analog supply, VSP and VSN, powers the high-voltage input section. The substrate of the IC is
connected to VSN; therefore, it must be connected to the most negative potential.
The low-voltage analog output supply, VSOP and VSON, can operate within the high-voltage supply boundaries
with two minimal limitations:
1. The usable range for VSON is from a minimum 5V below VSP to as low as VSN. This 5V provides the
headroom for the output supply voltage of +2.7V to +5V. Even with less than 5V supply, this voltage
difference is required for proper operation.
2. The common-mode control input, VOCM, requires a voltage at least 2V from VSP, in order to support
internal rail-to-rail performance.
These limits may only come into consideration when using a minimum supply or an extremely asymmetrical
high-voltage supply. In most practical cases, VSON is connected to the ground of the system 3V or 5V supply.
VSOP can be turned on first or can be higher than VSP without harm, but operation fails if VSP and VSN are not
present.
Observe the maximum voltage applied between VSOP and VSON, because there is no internal protection. This
consideration is the same as with other standard operational amplifier devices.
The digital supply, DGND and DVDD, can also be set within the boundaries of VSP and VSN. Only the positive
supply, DVDD, cannot be closer than 1V below VSP. It can be turned on without the analog supply being present
and is operational, but limited to digital functions in this case. The maximum supply voltage must be observed
because there is no internal protection. VSOP and VSON can be connected with DVDD and DGND, if desired.
Current consumption of the digital supply is very low under static conditions, but increases with communications
activity. Assuming no external load except the 20pF load to SDO, with an SCLK = 10MHz and a 3V supply, the
current momentarily increases by approximately 0.6mA when reading a register (for example, reading Register
14). With a 5V digital supply, the increase is in the range of 0.8mA. This additional current is only required during
communication; a larger bypass capacitor can supply this current. Driving current into SDO would further
increase the current demand.
VSN is connected to the substrate; therefore, the voltage at VSON or DGND must not turn on the substrate
diode to VSN. Use external Schottky diodes from VSON to VSN and from DGND to VSN (refer to Figure 50) to
prevent such a condition.
The PGA280 uses an internal chopper technology and therefore works best with good supply decoupling. Series
resistors in the supply are recommended to build an RC low-pass filter. With the small supply current, these
series resistors can be in the range of 15Ω to 22Ω. The RC filter also prevents a very fast rise time of the supply
voltage, thus avoiding parasitic currents in the IC. Connecting supply wires into an already-turned on supply (very
fast rise time) without such a filter can damage the IC as a result of voltage overshoot and parasitic charge
currents. Figure 50 shows an example of a supply connection using RC bypass filters. DVDD may not need
decoupling, but if the digital supply is noisy, a filter is recommended at C4 and R4.
NOTE
Rise and fall times for the high-voltage supplies must be slower than 1V/μs.
Copyright © 2009, Texas Instruments Incorporated
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