Do not confuse her with Hiltrude, wife of Luitfrid II.

Hiltrude, daughter of Charlemagne & Fastrada

Please see Charlemagne Project for Source Details

(b.787) [http://en.wikipedia.org/wiki/Charlemagne]
HILTRUD ([787]-after 800, maybe after 814). "Theoderadam et Hiltrudem" are named daughters of King Charles & Fastrada by Einhard[147]. Theodulf's poem Ad Carolum Rege names (in order) "Berta…Chrodtrudh…Gisla…Rothaidh…Hiltrudh, Tetdrada" as daughters of the king[148]. She lived at her father’s court until his death in 814. Wilhelm Kurze appears to have disproved the theory of the alleged marriage of Hiltrud to Eberhard [I] Graf [von Calw], a court official of Emperor Charlemagne[149]. According to Rösch[150], Hiltrud was the mistress (between [799/804]%29 of Richwin Count of Padua, brother of Richbod Bishop of Trier, who was at the court of Emperor Charlemagne between 792 and 814, and was the mother of an illegitimate son by him. He cites no primary source on which this is based and no reference to this has been found in the sources so far consulted. It is possibly based on onomastic speculation from the use of the first name Richbod. One possible illegitimate son: [http://fmg.ac/Projects/MedLands/CAROLINGIANS.htm#_Toc240955192] then is: [RICHBOD ([800/805]-killed in battle Angoulême 14 Jun 844). Abbé de Saint-Riquier 840/44. The Annales Bertiniani record that "Richbote abbas…consobrinus regum, nepos…Karoli imperatoris ex filia" was among those killed in 844[151]. It is possible, but not certain, that his mother was Hiltrud, as explained above.] :[http://fmg.ac/Projects/MedLands/CAROLINGIANS.htm#_Toc240955192] Note: Wikipedia thinks Richbod is more likely to be Charlemagne’s son through his last concubine Ethilind – who Cawley calls Adelindis, which would make him Hiltrude's half-brother instead

https://de.wikipedia.org/wiki/Grafen_von_Calw