I use Wheatstone bridge as a dual voltage divider to feed both inverting and non-inverting inputs of differential op-amp: Theoretically voltage can be calculated using voltage divider expression: Va = V1*(R2/(R2+R1) = 7.5 mV. Following the same reasoning Vb = 6.0 mV. When I use LTSpice to calculate these voltages for the bridge alone without op-amp, everything is fine. However, when I connect bridge to op-amp circuit, Va = 6.5 mV and Vb = 4.8 mV. If LTSpice doesn't give erroneous results, how should I calculate voltages Va and Vb?
Many thanks to @Andy aka for the hint.
Also, it would be useful to draw a common ground for the whole circuit. In this case it would be obvious that R4 is in parallel with (R7 + R8). The Thévenin resistance for this branch is 37.5 kOhm and Vb = 4.8 Ohm. Then the voltage between R7 and R8 is 3.84 V. According to ideal op-amp model, the same volatge is between R5 and R7. Now we can calculate Va: (Vs - Va)/20k = Va/30k + (Va - 3.84)/20k. ==> Va = 6.5 V.