How good are VHDL's random numbers?

I'm using VHDL's random numbers from IEEE.math_real, but how good are these generated numbers?

.... let's say compared to rand(...) from C.

Have there been statistical tests?

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Here is a histogram of a Gaussian distribution. Parameters:

• Random source: 2 uniform distributed REAL values generated by math_real.Uniform(...)
• Box-Muller transformation
• calculated using REAL
• Output rang: 0..4095 INTEGER
• 102.400 iterations

Classic histogram view:

As a point cloud:

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Here is a histogram of a uniform distribution. Parameters:

• Random source: uniform distributed REAL values generated by math_real.Uniform(...)
• calculated using REAL
• Output rang: 0..4095 INTEGER
• 102.400 iterations

Classic histogram view:

As a point cloud:

Gnuplot fitting results for f(x)=m*x+b:

m = -0.0000343906
b = 25.0704

In my opinion, both histograms have a high jitter.